Look, its pretty f--kin simple................. I'll explain it as if you are 10 yrs old
The taxpayers in his district pay a shitload of money to the federal govt......... the federal govt decides they want to spend that money....... RP votes against it (lets give it back to the people that paid it)............. the majority votes to pass the spending bill because they are not like RP, his vote lost, so since it passed anyway, he might as well get some of that money back to the people in his district that paid it in the first place, and that is done through earmarks. Understand?
If you were a tax payer in his district, would you rather the federal govt take your money and spend it on studying the mating habits of earthworms somewhere in California 1000 miles away , or would you rather at least get some of it back to your town where the money came from in the first place? Sure, it would be better if they didnt take it from you at all, but since they do, would you rather just have a congressman that said "nah, let them spend our money somewhere else" ?