Suppose I pass out notes at your school. Some notes are coupons for a tanning salon. A few say "You are the chosen. Wear a green ribbon in your hair tomorrow. This note is your ticket to the awesome concert".
The next day you wonder who will be coming to the concert. You look for people with the green ribbon. That means they got the concert ticket.
Your plasmid has a gene of interest and a marker. The marker is antibiotic resistance. If you got it, then you fail to die when the antibiotics come. If you fail to die that means you have the gene of interest too. Antibiotic resistance is a marker for having got the plasmid, just like green ribbon is a marker for having got the concert ticket.
To get the ticket but not notification about the ribbon, or ribbon without ticket the note must have been torn or damaged. To get a plasmid with gene of interest but not antibiotic resistance means the plasmid must have been damaged. If the plasmids were good when you added them there is not much time for them to have been damaged by random events. It could definitely happen but odds of that happening increase with time, and in the short term odds are against a damaged plasmid or a torn note.