There are two types of spec projects we never recommend, commercials and unpaid background work on any project. Spec projects are just that, speculative. They are made with the hope of selling them later on.
Do spec projects sell? Sometimes. But if you do a spec commercial that sells later, you have bought yourself a conflict. And if the project was done for no pay, the negatives outweigh any possible positives in the future.
What about a spec pilot? That could be worth your time if you have limited credits and have never worked on a TV show. You can’t put it in your resume unless it gets picked up, but you can put it on a reel, so get that in writing and get a copy.
We never recommend unpaid background work because you are likely to be cut and never get a copy or credits.