A chain tool is very important, because at some point you’ll need to replace or remove your chain, and you pretty much can’t get the chain off without one short of destroying it.
A good chain tool will have a solid handle, a good system for holding links in place while you separate them and a long enough level that you don’t need to apply an inordinate amount of force to push a pin out of a link.
Sure, you might have a chain tool on your multi-tool, but have you ever tried to use it? That one’s there as a last resort, not to be used when you’re actually planning on replacing or maintaining your chain. Trust us, a good chain tool is worth its weight in gold.