For standard size chickens pullets (less then 1 year old hen) is smaller. so it's usually better to go all male for meat type birds. A Pullets uses its feed intake to produce eggs not meat.
For some of the larger birds like the cornish cross, the females get big enough you can use the as cornish game hens
Straight runs are usefully for:
Some times by ordering a straight run you can use the pullets for eggs and males for meat. But that depends on the breed they are, for instance red sex links males are about 1 to 1/2 pound lighter when dressed after butchering.
If you are looking to hatch you own eggs then straight run will get you a few meat birds and a few hens for your breading stock, and a choice of which rooster to keep for mating.
They are cheaper since they don't have to pay someone to sex the birds. and by when it's time to lay eggs or butcher the males the adult charistercs are present and you can sex the birds then.
Tom