For sales and marketing data.
Some companies ask for a product registration for warranty purposes, while some states have laws to protect the consumer for warranty coverage regardless of any type of registration. You just have to prove you bought it.
They may disclose the purpose of registration, some companies might offer an extended coverage for frame breakage or something like that. Some companies honor warranty after resale - transferable - others do not.
I can't advise you specifically on Giant. Yep, basically I know nothing.