no, the question was, “why” do you need to do that? what is the result?
I’m just trying to tell, that there may be many reasons to do that (for example, I have no idea why somebody need option #60, but I can tell why I need option #50).
My story is long and not interesting. But if you wish… (sorry for my bad english)
My ISP does not give static IP addresses. But I don’t want to change my IP. The solution is simple: in DHCP protocol the client can (and most DHCP clients do) tell preferred IP (usually this is previous IP, which client remember). And, if this IP is valid (exists in DHCP server pool) and not used by somebody else, then DHCP server can give requested IP address (or can ignore this option, depending on server implementation and configuration). This works with my ISP: I can take back my IP even if it changes. But to do that, I need to modify option #50 (dhcp-requested-address).
My problem may look silly… But, this is what I can do with another router (and also can with linux and even windows), and can’t with mikrotik. 
Yes, I know this is wrong way (the better was to get static IP from my ISP, but he does not provide such service).
Yes, I know that sometime I loose my IP anyway, but my task is only to minimize number of changes if I can.