What is the difference between 'take a walk' and ' walk'

Why do I have to phrasal verb 'take a shower' or' 'take a walk' when we can use 'shower' and 'walk' What is the difference between 'verb' and phrasal 'verb' in meaning?

In many cases such as with "take a shower" and "shower" there is little difference in meaning. "I am going to shower" and "I am going to take a shower" are equally valid and with no difference in meaning, although for AmE speakers I tend to think "take a shower" is more frequent.

With "take a walk", although it isn't an absolute rule, there is a difference from the bare verb in that it tends to emphasize walking for its own sake: "I'm going to take a walk to the store" has a connotation that you are doing so for exercise or to enjoy the scenery, although you probably will buy something also. When walking for a purpose the plain verb is more common: "I'm walking to the store" implies that your main purpose is buying something, and perhaps you are emphasizing that you're not taking the car or bicycle.

"Take a walk" does also have the "go away" meaning that @Joe-Blow ascribes, but -- again speaking only for AmE -- it's not the first thing that comes to mind. If you want to have that meaning, AmE speakers will tend to say "Take a hike" instead.

Note that

take a walk!

can be an idiom. It essantially means "go away" and is the same as "give me a break" or "get lost."

Confusingly, of course the words "take a walk" can simpy mean you are taking a walk, and have no connection to the slang phrase.

I think the origin of the idiom is something to do with the legal/prison system; you can easily look it up.

Note that Joseph has made the outstanding point that "take a hike" is another version of the idiom, and it's more popular in AmE.