It is amazing how many spell casters get through fights on nothing but cantrips and good luck until the time for the long rest happens. While that works, you do want to use your spell slots otherwise they just go to waste. In the case of Witch’s Bolt, you should save it for big fights against the tough bruisers your DM sends at you. The large damage, coupled with a few fighters or spells to keep a creature from moving out of range, can be very helpful for you and your party. Chances are you don’t want to be trading blows with the Mimic or the large gnoll boss you’ve found at the end of the dungeon when everyone is bloodied and low on resources. The Witch’s Bolt Spell can even the odds, if it works. It’s a big if, but it can make sure that combat slides in your favor and then you can take a long rest.