Last wish gives might

Pride gives concentration

Would anyone please ELI5 on characters / mercs you want either of these items on?

I don't think there are any builds were last wish or pride are best in slot. When you stack off weapon ed it has diminishing returns.

Fana is usually better on physical damage characters. And if not that then reapers toll.

Pride is good with a Faith PVM bowazon

Lastwish is pretty good on an act5 merc especially if you're doing a lot of ubers

On a PVM WW barb with very endgame gear, I like to run act5 merc with plague mainhand and lastwish offhand. This is pretty much the only practical way to get cleansing aura, which I think is really underrated and better than an act2 merc with insight. You can always have enough mana with godly enough gear, but without cleansing you have no good way to mitigate decrep or other curses cast on you