Why do we walk on "dry land" when we should be just walking on "land"?

land |land|

noun

1. the part of the earth's surface that is not covered by water, as opposed to the sea or the air.

Many writers in countless books and various writings use the terms "dry land" to describe earth's surface that is not covered by water.

"They walked on dry land."

Why, when simply, "land" will suffice according to its definition?

Do we call the earth under our oceans, or any body of water, land? If so then I would agree that using "dry land" is not redundant, but by definition it is.

Building on Josh61's answer, the phrase "dry land" as an idiomatic expression may have come from the book of Genesis (1:9-10) (as well as other references throughout the original Hebrew).

King James Bible (1611)

(9) And God said, Let the waters under the heaven be gathered together unto one place, and let the dry land appear: and it was so. (10) And God called the dry land Earth; and the gathering together of the waters called he Seas: and God saw that it was good.

Wycliffe's Bible (late 1300's)

(9) Forsothe God seide, The watris, that ben vndur heuene, be gaderid in to o place, and a drie place appere; and it was doon so. (10) And God clepide the drie place, erthe; and he clepide the gadryngis togidere of watris, the sees. And God seiy that it was good;

Septuagint (3rd century BCE)

(9) Καὶ εἶπεν ὁ Θεός· συναχθήτω τὸ ὕδωρ τὸ ὑποκάτω τοῦ οὐρανοῦ εἰς συναγωγὴν μίαν, καὶ ὀφθήτω ἡ ξηρά. καὶ ἐγένετο οὕτως. καὶ συνήχθη τὸ ὕδωρ τὸ ὑποκάτω τοῦ οὐρανοῦ εἰς τὰς συναγωγὰς αὐτῶν, καὶ ὤφθη ἡ ξηρά. (10) καὶ ἐκάλεσεν ὁ Θεὸς τὴν ξηρὰν γῆν καὶ τὰ συστήματα τῶν ὑδάτων ἐκάλεσε θαλάσσας. καὶ εἶδεν ὁ Θεός, ὅτι καλόν.

(9) And God said, Let the water which is under the heaven be collected into one place, and let the dry land appear, and it was so. And the water which was under the heaven was collected into its places, and the dry land appeared. (10) And God called the dry land Earth, and the gatherings of the waters he called Seas, and God saw that it was good.

Original Hebrew (vocab list)

(9) וַיֹּאמֶר אֱלֹהִים, יִקָּווּ הַמַּיִם מִתַּחַת הַשָּׁמַיִם אֶל-מָקוֹם אֶחָד, וְתֵרָאֶה, הַיַּבָּשָׁה; וַיְהִי-כֵן.
(10) וַיִּקְרָא אֱלֹהִים לַיַּבָּשָׁה אֶרֶץ, וּלְמִקְוֵה הַמַּיִם קָרָא יַמִּים; וַיַּרְא אֱלֹהִים, כִּי-טוֹב

(9) And God said: 'Let the waters under the heaven be gathered together unto one place, and let the dry land appear.' And it was so. (10) And God called the dry land Earth, and the gathering together of the waters called He Seas; and God saw that it was good.

A distinction is made here between יַּבָּשָׁה, the word that's translated to Greek as ξηρά and then to English as "dry land", and אֶרֶץ / γῆν, which is the typical word for "ground, land, Earth, ...". As far as I can tell, the former is only used in biblical texts, whereas the latter is part of modern Hebrew. Also, According to google translate, ξηρά literally means "the dry", rendering the first part of verse 10 "And God called the dry, 'Land'."
Edit: As @oranja corrected me, יַּבָּשָׁה is used in modern Hebrew, although mostly as the translation of continent rather than the biblical "place that is not water".

Some other uses of the word יַּבָּשָׁה include Exodus 14:29 ("But the children of Israel walked upon dry land in the midst of the sea; and the waters were a wall unto them on their right hand, and on their left."), Joshua 4:22 ("Then ye shall let your children know, saying , Israel came over this Jordan on dry land.", and Jonah 2:10 ("Then the LORD commanded the fish, and it vomited Jonah up onto the dry land."), among others. In all cases, it's used to contrast with the waters of the sea (or river).

Thus the phrase could very well have been in use as an idiomatic expression long before the first attestation to it in the 13th century origin that Etymology Online cites.

Dry land (noun [U] UK US ) is a fixed (idiomatic) expression generally used in contrast to sea.

• land and not ​sea or ​water:
  • We ​sailed for three ​days before we ​saw ​dry ​land.

(Cambridge Dictionary)

According to etymonline it is a very old expression:

• Dry land (that not under the sea) is from early 13c.

• I agree it may sound redundant, but it is a very old expression (13th c.) which probably became popular during the centuries of explorations by sea.

Picture it from the sailor's perspective. There is a lot of "land" on coastlines that is not dry, and thus not too useful for a sailor. Rocks visible at low tide would be "land" but they would also tear the bottom out of your boat at different times of the day. You can't stay there, and you won't find food or trade good there. Dry land is above the highest tides and thus much more useful. The old sailing call of "Land Ho!" was just as much warning as announcement, as the ship would not yet know if the land was useful or not.

[Hi, terribly sorry for making a new answer, but unfortunately I can't comment on @Bobson's answer directly. Adding just a couple of notes on this matter:]

Actually, יַּבָּשָׁה is used in modern Hebrew to indicate:

1. a dry land, an area of ground which is not covered by water
2. one of the six areas on Earth, which are neither a sea or an ocean

* free translation from https://milog.co.il/%D7%99%D7%91%D7%A9%D7%94

The first refers to the biblical origin, I believe. The word is less frequently used in this general meaning in modern Hebrew. However, the second indication is widely used. So יַּבָּשָׁה basically means a continent in modern Hebrew.

As I mentioned, יַּבָּשָׁה is less commonly used in its biblical sense nowadays, to the point where the connection might not occur to an Hebrew speaker immediately. Anyway, it is there: The root of the word - י.ב.ש - is attached to the meaning of "dryness"; יֹבֶשׁ means dry, לְיַבֵּשׁ means to dry, and so on.