As a software engineer, the nicest thing is that the whole programming ecosystem integrates with Linux. Git, SSH, Docker, you get natively in your OS. Even dumb shit like file path separators, line-endings, file permissions. Most programming languages make the assumption that you're on a UNIX system (Linux, macOS, BSD).

Aside from that, Linux is fucking awesome as an SE, because everything is open-source. Find a bug in a program you use? You can fix it, if you want. Want to learn how a specific program works? Just look at the source code. Or its config file. Or its logs. Everything wants to teach you about itself.

And personally, I also just love the usability. The built-in file manager, terminal, PDF viewer etc. are good. The built-in text editor is no IDE, but it's up-to-snuff with Notepad++.
And I'm making these blanket statements despite there not being one built-in anything. You can choose between multiple GUI bundles (so-called "desktop environments"). From a minimal DIY setup (i3wm etc.) all the way to maximally feature-rich goodness (KDE). You don't have to use the same limited setup as your granny uses to launch a browser. You can customize everything to your needs and you get tons of power-user features.