Gives the owner time to take action: lie down in a safe spot, relax, drink water, eat food. Etc.
By being warned, the owner may be able to even prevent a full on seizure.
I’m not sure what the dog is trained to do if there is a full blown seizure. I don’t know if they just protect the owner or get help.
My daughter had seizures and wanted a seizure dog.
I saw a man having a seizure on the sidewalk at night in Philadelphia. He would have been vulnerable to having his phone and wallet stolen if not for the good guys who stopped to help.