All agents do not require this, and as you see, the reasons vary.
Virtually every complain that clients have about real estate agents concerns communication. Ethics violations, bad behavior, incredibly (thankfully) rare. But, oh, the complaints - s/he kept showing me houses out of my range, didn't call me often enough, I had to find the houses because s/he would only search evenings, whatever!
Point being that with an agreement, you have a formal understanding of what each other's responsibilities and obligations are. I will tell you that for centuries, we have had exclusivity agreements with sellers, but only recently have we been doing this with buyers.
Aside from that, there is the problem of dual agency. Your agent represents you, but so does their Brokerage. If you elected to buy one of that Brokerage's listings, the Brokerage would probably be a dual agent, and if there's a problem in the transaction, attorneys really like to see how that possibility was explained to the buyer and seller before the transaction began. So buyer agreements almost always have some clause concerning potential dual-agency situations.
Oh, yes - one more thing - we do like to know how we're going to be paid. Typically, we rely on the co-brokerage on listed properties, but we'd like to know how we are to handle non-listed properties and how we are to be compensated if you buy one.
All the best, Maria!