Baseboard Beautification Questions

I was trying to improve the paint job on the wall near my living room's baseboard. I ended up prying the baseboard off. Naturally re-installing it leaves a gap that is not noticeable from afar but is noticeable if one looks straight down. The walls I haven't touched look much tighter and "well-sealed."
Does sealing this gap help insulate the house heating and cooling wise, and also keep bugs out? So I should not neglect it and try to seal it?
How does one close up the gap between baseboard and wall?
Is it merely a careful coat of the baseboard paint between the top, back edge of the baseboard and the wall?
Or is there some kind of "baseboard top" sealant that one can apply lightly so as to eliminate the black-holed gap between baseboard and wall?
I'm a great home mechanic but terrible at making things look pretty.
All constructive advice is welcome.
TIA
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It is called latex caulk. Buy the higher quality latex with silicone (paintable). Gaps larger than 1/8" may require two or more applications to really seal the void.
The DYI guys best friend.
Colbyt
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That solves it. Much obliged. :-)
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