I'm happy to help!
A DAC converts digital audio to an analog signal (DAC stands for Digital to Analog Converter). These take your digital signal (your audio file, CD, Spotify music stream, etc..) and turns it into an analog signal for you to hear with your headphones or speakers. Vinyls are different in that they do NOT produce a digital signal, they produce an analog one, therefor a DAC would not benefit them at all.
Anyways, I would certainly
not recommend getting a DAC if you listen to primarily to vinyl. Now with that said...
DACs certainly improve sound quality, but NOT in the same way headphones do. Headphones vastly shape the audio you hear (bass, mids, treble...), while DACs alter things like resolution, detail, and transparency (transparency is how easy it is to hear every instrument in the sound stage). That's not to say headphones don't alter detail and resolution, but DACs do this in their own way. The best way to understand this is to hear some and compare them yourself
A DAC
would improve the sound quality of your Spotify music, but if you're going to spend money on a DAC, using it with the lossy audio from Spotify would not yield as good of results compared to if you used lossless audio. It wouldn’t be a total waste, but be aware than when using higher-end headphones and audio equipment, the imperfections of lossy audio begin to become more audible for many people.
If you have no idea what lossy and lossless audio is, I'd suggest reading
this article, as it is a very important part of the "audiophile world” Don't worry,
it's a pretty simple concept.