I see that frequently as advice, and it does make sense for some. Personally, I'm just not a renter. Lensrentals charges ~4% of the lens cost for a week rental. Shipping is not free, either. As an example, I bought the RF 100-400 thinking it would be a great travel lens for a trip to Italy. I'd have needed a 3-week rental and that plus shipping would have been 21% of what I paid for the lens buying it new. I'd rather not 'waste' the money. As I said, personal decision. The reason I could see for it is if you needed something for a short-term, one-off project it would make sense to rent it.
I benefited from the other side of that, my MP-E 65 was bought used from a lawyer who bought the lens for a project then didn't need it any more. I paid $500 for a barely used lens that was selling new for $1000 at the time.