If one cares to read in the OT, there's very good reasons why a man was permitted to have more than one wife. In one particular case, it was where a man had died without issue (probably, without having a son). His brother was to marry the widow.
(1) The first child then born of that union would be credited to the deceased brother, to keep his name in Israel and to inherit his portion of the land.
(2) The widow would also be provided for - remember, no widows' pension arrangements in the OT.
Remember the story of Onan - who "cast his seed on the ground"? I'll let you "research" or even just plain re-read that.
And the man's "duty" to his second or subsequent wives was to be exactly the same as to the first wife. She was not to lack food, raiment or care - specifically, sexual pleasure. (Certainly in modern Judaism, did you know that sexual satisfaction is considered to be the right of the wife - not the right of the husband?)
David married his second wife, Abigail, wife of Nabal the Carmelite, who had just dropped dead, to protect her in a time when women had few protections. Don't know why he married the others - lust? political unions? I have no idea how he could provide equally for al his wives, much less his unfortunate concubines.
And just because he had all these women, don't be thinking for one second that that was God-sanctioned. God's idea is one man with one wife. Says that at the beginning of the bible, and is repeated in the epistles.