There's something you can do with the gloom stalker ranger and the assassin rogue but I can't remember what it is

I'm curious what's up with Paladin-Barb

What do they really offer each other besides hitting things?

Thief is pretty good with most things just for the bonus action alone. But if you are a dual wielding martial that bonus action becomes even better since it can always translate into damage. I haven't played a ranger yet though and I'm not sure if hand crossbows are used frequently with them.

I bet it's range and sneak attack. I play similar in my DnD campaign. I like rogue fighter better in dnd. It was my favorite character, but so far crossbows seem weak in BG3 compared to dnd. I'm only in the 1st area in BG3 so maybe they get better.

I would guess the order just indicates they started as a fighter, not that they are mainly a fighter. So a 1 level dip into fighter first for heavy armor and a fighting style, then melee warlock from there makes sense to me.

Otherwise, maybe it's just people thinking that fighter 5/warlock 5 gets you just as many attacks as a full fighter, and also fireballs/hex etc

Consider some people use mods so you can multi class up to level 20