Endless or not?

[Marth]

What is endless mode that can be toggled on and off?

is there any pros and cons?

[Dead Inside]

Endless mode is for fun?

You do not get any new unlocks or anything for doing it. But sometimes, a build is a lot of fun and some people enjoy the challenge of getting to as high of a level as they can.

[North]

if you toggle on endless mode at the start of a run you'll be able to continue past lvl 20 and still get the completion mark for the character you used

[WabbaCat]

Pro: The satisfaction of using a potato who turned out well. Or torturing yourself with a rotten potato on harder waves just for the heck of it? You masochist...
Seriously tho... maybe you just stubbled on a build you really like and want to see how far it can go.
Con: Time consuming. No rewards. Not intended gameplay but rather a byproduct of the existing gameplay.

Extra Pro: Street cred.