Ok, I thought of one reason to maybe start seeds in a small cup versus directly into the end goal pot when there aren't "soil in the pot is too rich for seedlings" type reasons. If I'm starting several, say 5 or 6, then that many five gallon pots (or even three gallon) take up enough space I'd need to run most all my grow lights. If I do five little mini-Solo cups, I could cram them all under a single smaller light and save a few bucks on electric. Only for a week or so, though, so pretty flimsy reason.