An earlier protocol, Reverse Address Resolution Protocol (RARP)  was proposed to allow a client to determine its IP address, given that it knew its hardware address. However RARP had the disadvantage that it was a hardware link level protocol (not IP/UDP based). This means that RARP could only be implemented on hosts containing special kernel or driver modifications to access these 'raw' packets. Since there are many network kernels existent now, with each source maintained by different organizations, a boot protocol that does not require kernel modifications is a decided advantage.
BOOTP provides this hardware to IP address lookup function, in addition to the other useful features described in the sections above.