The point is to make everyone have the same size window therefore nobody stands out. We’re all Spartacus as it were. Of course you can just click maximise if you want.

When maximised the size and resolution of your screen can be determined and used as a piece of data among many to uniquely identify you and attempt to figure out your identity. Depending on what you’re doing and which sites you’re using this may or may not be a concern.

For example, when I use my university’s website, they already know who I am. I log in with an email address uniquely tied to me. So maximising the window then, to me, doesn’t really matter. But if I’m browsing news articles from websites hell bent on bombarding me with adverts, cookies, and trackers then I’ll stick with the default Librewolf sizing in an attempt to blend in.