Hi there, I hope you don't feel pressured to take the first school that offers, without considering all your options!
There should be a deadline to accept their offer. In the meantime, they tend to have events for prospective students to come check out the campus, meet with the professors, basically interview them for whether you'll give them the job of being your school. All the stuff they don't have time to do for literally every applicant or person who's thinking of applying. The pool of people they offer to is more manageable for them to entertain in this way. They all court you, meanwhile you dig even deeper into the faculty and their personalities, the university culture, etc (to an extent that you also couldn't do for every school in existence but now can for this smaller pool who has extended offers to you).
Keep a timeline of when to expect to hear back from each school and when decisions to accept are due, and take all the time you need to decide. A doctoral program is a huge commitment of ~4-7 years, so there's no reason to shortchange yourself on your decision-making time!