basically you are asking what kind of advantages do these alternative terminal emulators have over the default.
well, just look at what everyone here is writing about their choice.
in part that depends on what the default is because on linux you get a variety of choices. there it often comes down to preference. the defaults change over time and some people prefer to stick with what they had.
if it's not that then it's some kind of specific feature. it can be font choice (better default fonts, or an easier way to change the font), tab support or something else. it's not that one terminal is objectively better than another, but a few features that are important for one person but not for another.
on macos i occasionally had issues with unicode support for foreign languages in the default terminal.app. so i switched to iTerm. iTerm also has tabs and tmux integration, which are nice extra features.
btw tmux is a terminal multiplexer offering features that no terminal application has. the primary one is that it keeps running if i close a window and i can reconnect later. that is very useful for remote connections, but also locally it means i can log out without having to close all my terminal windows. keeping that state for me is very important to make it easier to switch between activities