I don’t want my guests to be able to access my home server or Omada controller for example, or spread malware (their phone may have malware without them even knowing). Also, I give the guest wifi to people other than friends, like contractors. Phone reception is horrible at my house so I give them the wifi so they can use wifi calling.
I used to have a Netgear Nighthawk router/AP I bought from Costco, and if I remember correctly, its guest network automatically isolated guests from other guests. This router didn’t support VLANs so I think it was just a bunch of firewall rules.