Employment offers typically consist of things like health insurance, vacation, changes to the commute, etc that do require consideration in addition to the salary. It’s not unheard of to need time to process the offer, discuss with family members, etc.
I had a job offer once that looked great until you realized that a 10K salary increase was going to result in me getting paid 6K less per year because they didn’t contribute to the health insurance (which was about 16K per year in premiums for my family)
So, thank them for the offer, let them know you’re considering it and will respond “soon” but don’t make any hard commitments.
It’s also not unheard of to let other prospective offer-givers know that you’re on a clock. “Hi there, I really enjoyed interviewing with you and your staff, and really like x, y and z that you described about the opportunity. I would love to be considering an offer from your company along with another I recently received, and I have until X to return my response to the other company. If you could let me know the status of my application with you, it would be much appreciated!”
I’ll caveat all of this by saying that if a company rescinds an offer because they don’t want you to hear from other companies, then you probably didn’t want to work for them anyhow (unless you’re desperate and just need employment.) Having multiple job offers during a time of low unemployment is something that most companies should expect, and professional companies shouldn’t react negatively to it.