I keep hearing "kill the males" but why?
Because males give low yields of low-THC flowers that are not pleasant to smoke.
Growing indoors is "expensive" in terms of energy, and labor intensity. You have limited space under the lights, and to use it most efficiently you don't want to waste any on low yield low potency males.
Meanwhile the pollen from the males will get EVERYWHERE, ruining any female plants that you are looking to grow.
1. Can you harvest/smoke the males?
Yes, you can, and in fact the growing tops of male plants can have a decent amount of cannabinoids in them. But overall yield/quality is FAR less than females, and if you can only grow a few plants, you certainly don't want males.
2. Can you harvest/smoke the females that have seeds?
Again, yes you can, and historically (ie before the mid 1980s) most pot sold in the USA was seeded. Globally speaking, most of the hash on the planet is made from outdoor grown seeded plants. Quality of seeded pot can actually be excellent.
The problem is that, plants making seeds will give less overall yield than seedless ones, and a seeded final product is simply less desirable both to use (you have to pick out all the seeds before smoking) and to sell (because much of the weight of the final product is seeded).
The seeded stuff may not be bad, but given the choice, you probably want to grow and smoke seedless.
3. If both are yes, then why not let them grow naturally, harvest the seeds, then the leaves & smoke them both?
See above.
If you are growing outdoors for personal use, in an area where there are no legal consequences to growing, and you have plenty of space, it may be easiest just to plant the seeds, and let things go, and harvest seeded flowers in the fall.
But if you're growing indoors, you don't want to waste space on males.
If you are only allowed a certain small number of plants, or have limited space, again, you don't want to waste it on males.
And if you want to sell your buds commercially, seedless is the stuff that is worth the most, and given the choice, that's what you are going to want to grow.