You don't grow crops in a forest, you grow them in fields. They weren't slashing and burning in forests, they were doing it in open fields so they could get rid of the weeds and refuse that had grown since the prior year's crop. Once that dried they set fire to it leave a clean field for sowing their crops.
Many farmers burn their fields and pastures today, it's the same principle.
Nothing much has changed, really.
In Linn county Oregon 'Grass Seed Capitol of the World' they burn over the grass fields
so that the seeds produced in the newly sown fields are all of the same variety.
Can't have the wrong variety of grass seeds in the bags, can we?
That would be false advertising!
