The coupons offered in such offers are often for the first month only. The prices might sound cheap but after the first month you will have to pay the normal price. If it's a lifetime discount then perhaps the host has a different kind of business strategy. They might for example be looking for new customers and popularity with affordable discounts.
Example: Cheap prices → new customers → new positive reviews → more popularity → more sales → more profit.
The best deals does not represent best quality or best experience you will get. Therefore as above suggested, it is essential that you find legit reviews and to look around to find the host that fits your needs. You dont want to signup with a doubt in your mind because thats like shooting your self in the foot... coz its going to be a pain to keep moving etc
every webhost has there own special offers! all those coupon are made to gain new custumer. marketing strategy i guess. everybody compete with all the webhost in the world so wich one would fit your needs & is it a good quality & reliable. sorry if my expanation really bad
It could be a good idea to ask providers about their special offers directly. Don't be afraid to ask about sustainability of the offer long term, number of resellers per server, etc. This could make it easier (along with thorough outside research) to spot the good deals from the bad ones.