I think the main use of marines is to create beachheads. Land them on an undefended (no fort) province and siege it down. Then you can bring in your regular army by docking transports.
Also, they use sailors instead of manpower to reinforce, which is generally an advantage. Especially if you kept your home territory small and invested into overseas trade companies and colonies instead.
I keep hearing this beachhead argument, but it truly holds no water (pun intended).
If the problem is landing on an enemy stack, then a marine army will still absolutely get wiped on landing, they are too few and too weak to be more efficient than landing a normal stack.
Now lets analyse the niche situation where the enemy isn't garrisoning the coastal province, but is moving an army there, so you need to land fast before they are able to garrison it.
Then maybe, in this situation, a Marine army can land faster to avoid falling into the enemy stack whereas a regular army would get intercepted... However, the marine army will NOT be able to hold the beachhead long enough for the main army to land, so they will still get wiped once the enemy force gets there and the regular army will still get intercepted after. So this was all for nothing.
The only situation where a Marine army can be useful would be in a scenario where you are like the Netherlands fighting a war in Japan, so your ships are taking attrition while in the coasts of Japan and you need an army to quickly occupy one province so your ships stop taking naval attrition. Still, adding an entire new unit type so you can take one fewer month of naval attrition in a naval transport seems beyond niche.
As for "costing sailors being benefitial to low manpower nations with a trade company based empire" is a trap idea.
I too belived this as well during the dev diaries, and i assumed Marines would serve as minor manpower boost for nations like Portugal and the Netherlands. Then i actually played the game and realised that manpower is always much more easily available than sailors, and using sailors as an alternative manpower source is a quick to get 0 sailors almost immediately and then you'll be unable to afford more ships, which is extremely crippling for a global trade based empire.
One marine regiment is as sailor costly as 5 heavy ships. 5 heavy ships can be the decisive difference between completely dominating or utterly losing the naval war in a conflict. Whereas +1 marine regiment won't make the slightest of difference even in a HRE OPM 1v1 war.