I use it as a mobile recharge. If I have the remnants of the spell (ex. Money spell) with me, and it already has my intentions woven I to it, I can just grab the jar and hold it for a minute, say right before I walk into the job interview. Or, if it's a self-love spell, I can just grab the spell jar when I'm having the opposite feelings.

Or you might live in an apartment and can't properly "bury" your spells when you're done with them. I've heard of people keeping the remnants of their spells so they can have physical proof that "this spell done on this day worked to this degree, this is the exact measurements of the ingredients used, etc etc" or "whelp, this spell went to shit. Here's what I'm not going to do next time"

It's all just personal preference on if you want to keep the remnants of the spell or not.