Familiars are okay. Well... Some of them.
Birb can be useful to occasionally get a Blind on an enemy (Works well with Beastmaster Ranger also using the Birb companion. Just have a murder of crows pecking at enemies)
Imp/Quasit/Shovel is really good because permanent invisibilty means it can run around the battlefield with impunity, ignoring turns and just sit next to targets to let Rogues sneak attack them.
Besides that... Ehh...
They can scout, but they won't trigger scripted ambushes (Like the Beholder in the Underdark) and combat triggers are janky and they can sometimes put your party into combat while way out of position.
They can activate switches... But besides that one in Grymforge by the Shar statue most levers are placed so a ranged attack can use them (Also, familiars don't have Perception so can't find hidden switches like the one that opens the walldoor in the Taxhouse)
Something that could be nice is if they could be used as the vector for spells. Being able to Thunderwave from a birb that's flown around to an enemies flank. Or using Shocking Grasp without having to run your squishy caster into melee. Or a Imp dive bombing into a group to cast Arms of Hadar...
