New granite countertops big circle spots on.

Yes, for sure you should put granite sealer on the countertop of you want to keep it stain-free. I would recommend you to apply simple, chemical free and waterproofed granite sealer as soon as possible. Here is the list of the best granite sealers https://graniteprotect.com/ with all necessary details. If you don't want to pay installer, then you can perform this sealing job yourself with the help of a family member.

They suggested a dryer because they thought the stone had absorbed a liquid and was causing the stone to darken. This is common on unsealed granite. If you put a sealer on the counter tops however, whatever it is you see there will remain under the seal. I would ask the installer to come out and take a look at it. If they are unwilling, the Home Depot has a cleaning kit sold in the kitchen counters area of the store made by a company you can call and they will ask questions about the problem and provide advice on how to fix it. They are extremely helpful with free suggestions as well as stuff you may be able to purchase to fix the problems. In any case, granite is sensitive stuff and while it is physically durable it seems to be extremely sensitive to stain and etching and learning about proper care, cleaning, and dos and dont's will save a lot of heart ache and time for you down the road. (For instance oil absorbing into the stone is a several day ordeal to fix). Good luck with getting the results you want. If you did find a specific solution please share it here.