Are there any online search engines for buses in Southeast Asia? I wish to plan a bus trip in Thailand

Most websites for regional bus companies are in Thai and payment is often limited to ATM, bank deposits and such, not international credit cards.

Most list buses schedules from A to B, without much detail on other stops enroute. You often have to guess at intermediate stops and then compare timetables to see if bus numbers match.

Long Haul bus routes tend to pick up at A, with perhaps a stop or two close by and then head for B, with a mid-trip break at some food stop. As an example a bus from Chiang Mai to Bangkok will likely stop at Lamphun and maybe Lampang to pick up more passengers and then head for Bangkok. They will take a food / stretch your legs break at Thoen or Tak.

Is there a need for knowing where it stops or simply curiosity?

12go asia has it all, stops and maps. you can hop off anywhere, but hop on mostly in provincial centers or large towns. unless you solve local logistic, there is no point in this kind of hops. how do you want to go from the provincial bus station, and where? they don't rent motorbikes there on every corner, actually don't rent at all. no taxis. no tuk tuk neither, mostly. imagine a wat in 35 km away from town, how do you get there? how do you get back? this issue is more difficult than bus hop on hop off :) especially provided most of long hauls really go bkk - distant place with no stops, interprovincials will stop anywhere you ask.