What are the benefits of breaking your first nether portal?

Solution 1:

Any automatically created portal is placed at a nearby location that's open air. So, the Nether side of your portal might be not at the exactly corresponding coordinate location but significantly displaced.

If this happens, then when you build more Overworld portals, you might find that your Nether portal now links up to a newer, closer Overworld portal rather than the one it used to. This means it isn't useful for fast travel, and might unexpectedly “strand” you.

If you rebuild portals so that they are in matching Overworld/Nether locations, digging into the ground where needed, then additional portals won't interfere with each other.

There is no short-term urgency in doing this — it is just something to do, or at least plan out, before you start building tunnels or transport hubs between particular portal locations.

Solution 2:

Consider that, when you spawn in the Nether for the first time, you could be unlucky enough to end in the middle of a lava lake, or on a platform hard to reach, or just in the open world without any kind of shelter, face to face with a Ghast. Those are definitely not "right spots".

Moreover, keep in mind that teleporting to the Nether is a good way to travel long distances exploiting the 1:8 coordinates with the Overworld. What you may want to do is create a long tunnel with several Nether portal to ease your travels, and it's better to decide where to build a structure like that.