Similar reasons like everybody else who have posted on here, one important factor for me was the flexibility of dipping in and out of Entertainment, Cinema and the occasional Sports Passes and more importantly for me using vouchers codes on your account.
Where over time you will be surprised how much you can save compared to the standard monthly subscription pass costs.
Another thing that i like with the NowTV service is that you can take your small NowTV box to a family members or friends house or other possible scenarios where they don't have the NowTV service, where it is more difficult with the SKY Box and Satellite Dish 
Don't get me wrong, there are other features and functions on the Sky Satellite Service that i would like to see on the NowTV Service (just look at the ideas board), however i can understand to some degree has a business point of view why Sky needs to differentiate both services.
@Anonymous User
Hopefully when Sky rollout their proposed Sky IPTV service scheduled sometime next year and if you have sufficient internet speeds at your rented accommodations.
Then the Sky IPTV service might be a good alternative to Sky Satellite which your Landlord might consider/accept with no ugly dish attached to their property.
I believe this is one of many reasons why Sky are bringing out their IPTV service where they are losing potential customers who are not permitted to have a Satellite Dish installed at their property.
Or maybe stick with NowTV where you are not tied into any long term contract 
