Here is a reason you might want to keep that /24. Suppose you are a small ISP and I am your customer. I also have another larger provider. That larger provider is also your provider. I own a /21 and advertise it to my larger provider. You get that /21 from my larger provider. I advertise a /24 subset of the /21 to you. If you ignore my /24, then traffic for it goes to the larger provider and I pay him for the traffic, not you.
Regards, Jakob.
