Why do younger sons and distant heirs get such good marriages?

Obviously being heir/lord does matter in marriage politics but it seems to me that maybe not as much as I thought for whatever reason.

I mean Stafford lannister was negotiating with paxter redwyne for marriage between Daven and desmera which to me is kind of mind blowing because you'd expect Daven to marry some random knight or merchants daughter, not the only daughter of the lord of house redwyne. At that time daven was so low on the inheritance list and basically a background character for the lannisters, it seems very cruel to desmera that her father would marry her off to devon instead of a lord or heir. She is far too good for daven, and he has the sheer audacity to dare make fun of her freckles.

Also then there's egg who's prince but the youngest son of the 4th son of the king so even lower importance than Devon but he married lord backwoods daughter, and egg himself got pretty nice marriages for all his sons, even those who'll get nothing, with the daughters of powerful lords.