Why is it beneficial for bacteria to conjugate? (Horisontal gene transfer)
I am going to try answer it. Bacteria in general has different survival tactics. In general for bacteria the best way how to surviva is as much copies as it cans. So imagine situation, when bacteria is surround with others bacteria. Then the race will bega. All bacteria wants to survive with same tactic as i mentioned. Thats mean the best way how to archived this goal is make your replication fast.

So when you take plasmid with resitantion to antibiotics its advantige to have those, but when you actually is not exhibit to that antibiotic it slow down your chance to survive (because it slows down your replication, cell division)