Why is halogenation important?
Because it introduces functionality onto a carbon chain (sometimes selectively), that can be further elaborated.
Hydrocarbons, the ones we pump from the ground, are rather unreactive molecules. Of course you can burn them (and we do to supply energy!), but other than that they are rather inert.
Radical halogenation (or hydrohalogenation of olefins) allows the introduction of an halide group at a predictable position on the carbon chain. This