Is TSA Precheck useful to get through Sea-Tac airport?

It is 100% worth it, to me. You don't have to take off your shoes or remove anything from your bags even if the airport you are traveling through doesn't have a dedicated Precheck line. My home airport is ATL and it is a lifesaver there, in the domestic departures terminal especially, but even in the international terminal which does not (!!) have a dedicated Precheck line. My husband frequently travels through SRQ (Sarasota) which is a small airport and even there he says he often bypasses a line with Precheck.
If you travel only once or twice a year, it might not be worth it. The application is really not that big of a deal. You show up with required identification documents and are fingerprinted and photographed. It's good for five years.
You can google "TSA Precheck" and there will be a million search results that will tell you all about it.

Do you ever travel to Canada? I’d suggest a Nexus card. $50 and it comes with pre-check and global entry. You also get expedited border crossings.

Spot saver just moves you to the head of the regular line. Pre check puts you in a different line.

Yes, it is.

Precheck is definitely useful at SEA, and I expect at most airports (but not ATL apparently). Whether it's worth the trouble and cost to you, in money and trouble, depends on how often you fly and how much time you have to wait in lines at the airport.

I don't know where you read you had to go to a Staples store, but you don't. You can apply online and make an appointment at a nearby processing center for fingerprinting.

https://www.tsa.gov/precheck

There is a new program where some Staples stores do offer enrollment services. Three stores in the Seattle area offer this but there are other locations. You'll find out where when you apply.

https://www.identogo.com/staples

If you plan to travel overseas, I'd suggest looking into Global Entry. It's $15 more and is also good for 5 years. It will help you breeze through U.S. Immigration and Customs. It includes TSA Prechek.

https://ttp.dhs.gov/

I second recommendation for SEA Spot Saver. Daughter and family live in Seattle and we travel SEATAC often. Find it at portseattle.org.

The closest TSA Enrollment center for many of us is inside a designated Staples store. Some enrollment centers near me are temporarily or permanently closed according to the TSA Website. I received my approval 3 days after applying but my husband who applied with me same day ten minutes later hasn’t been notified yet going on three weeks since our appointments. Has anyone else experienced a delay like that?

Went into a Staples 8 miles from my home two days ago and went through the interview in 7 minutes or so. Leaving in 24 days, hoping to get approval so I can add it to my airline reservation. Will be looking for a separate TSA Pre-check line. Also maybe add SEA Spot Save (it's an app, too) or whatever it's called, in tandem with the above. Confusing, but necessary.

@Denny. You can use Spot Saver orTSA pre-Check. You can’t use them together.