Why Did The Raiders Sign Doug Martin?

I swear every time I see or read a headline about The Raiders this past off-season I can't help but say "Oh God Why?". And the headlines about Doug Martin signing with them is no exception.

Seriously, why? you have young running backs, and Marshawn Lynch (BEAST MODE) and still decide to sign another overpaid player. Do the Raiders just walk around thinking "ehh, fuck it, let's just spend more money on the useless talent we dont need".