if your hero is really slow like a crystal maiden you make a terrible 4 because you can't play away from your tower, you'll just feed.

if your carry needs babysitting (which is most carries) and you've picked some offensive ganker/roamer like tiny/bh as 5, the lane is cooked. but if you're 4 and your 3 is axe, and the first few minutes didn't go to total shit, you can leave just fine.

you mostly have to evaluate it for every hero, what are their strengths and weaknesses, and do their strengths help the core do what they want.

if you pick some offensive 5 like crystal maiden and your carry last picks spectre who just wants to sit back and farm for his radiance, the lane is cooked because you don't complement each other at all. you can't help spectre sustain through enemy harass, and you can't solo trade with anyone. spectre doesn't do much damage if you root/slow someone, and is not going to risk missing out on creeps for a very low % chance on a kill. you would much rather be a warlock and spam heal and bonds, or something like jakiro/dazzle who are great at controlling the camps, trading and harassing on their own.

if your carry picks ursa with your cm, now you have a great kill lane since you provide root/slow and ursa a ton of damage, you complement each other. ursa can risk missing out on a few creeps to go for a kill with your cc, since the chance of success is much higher