A leader key will expand the number of keys available to you. You can have maps like <leader>j that doesn't interfere with the default j map. There is also <localleader> that does the same thing. They're like having a sticky <ctrl> or <alt> but more comfortable. Read up on them (:h mapleader) and you will have more sun shine fun with Vim! For the record I use the comma as the <leader> key and leave <localleader> as the backslash since it's used rarely.
I started using ? as the leader (I don't do reverse searches). The other options (, \) always get in the way while programming. I might try the backtick, but it means that programming in languages that use backticks (Rust, I think) would be a pain.
The backtick is also used for for jumping to marks. I wouldn't recommend overriding that one. `. will put your cursor back to the last insertion point. '. will put you back on the last insertion line. They're quite handy. A common key that is used is <space>. There' is also using CAPSLOCK as a leader key, but it's not very portable since it requires configuring the OS. I think ? is fine for a leader key as long as you know that you're not going to use reverse search ever.
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