You can get flux core solder as small as 0.3mm diameter, and its certainly worth having some small diameter solder (<0.5mm) of the same alloy as your regular solder or solder paste when you are dealing with small passives and discretes and need to add a controlled amount of solder to a pad or joint quickly to avoid cooking the pad off the board or remelting the tack-joint at the other end.
Without suitable diameter solder, there's more fiddling around, and the risk of pad damage or is much higher: If the solder wire is too thick, its very hard to control and you often end up with excess that has to be wicked off, and paste is problematic when using an iron because you have to preposition the right amount, and its very hard to control how much ends up on the pad or joint and how much on the bit, so you are lucky if its right in one shot. When you are using hot air or bulk reflow, that isn't a problem as all the solder in the paste in the immediate vicinity of the pad or joint will end up on the pad or joint.
Beware of kits with precisely counted small passives and discretes. You *WILL* loose a few, its virtually unavoidable, and when hand soldering, you may loose some that stick to the soldering bit. While it *may* be possible to quickly scrape the part off the bit, by the time you've cleaned it up, it will have been badly cooked, and you don't know how badly its terminal metallisation has been eroded by the solder so reusing such parts is unwise. There's also a problem with unmarked caps - if one flips away, and you *think* you've found it, are you 100% certain it isn't one of a different value you gave up on finding earlier?