just a general question to theme

cheers..I was wondering if anybody would know why my site is getting lots of fake registrations…they cant go anywhere unless they fill out all profile data…so right now its like 20 fake registrations but they didnt fill out profiles..so in essence nothing shows on site until they do this…why would people do this..as there is no advantage..ex. putting some link in.. to get people ti click..etc…Im just curious if anyone knows why ..its useless on my site unless you fill out entire account profile…Thanks for any info..