It is not related to any other wireless service; you have to "choose" (you can use the one at home or this one when you plug it in). You pay extra for its service, usually from a phone company; think of it as how your cell phone gets wireless when it's not using 3G/4G.

Yes, it could replace your home's wireless but if you are paying for your home wireless anyway (I have cable, for example) then it will be an added expense.

No, it might not keep you connected wherever you go, only where there is a hotspot/wireless network you are allowed to connect to unless you have added service from a phone company.

I have an AT&T Aspire notebook I pay $50 a month for (and it cost me $40 from them initially) 3G/4G and I am connected wherever I go and can be my own hotspot when my home cable goes out so my husband and I stay connected. It works well in the car on long trips, out of the country, wherever AT&T phones work, in other words. I do not usually get my $50 a month worth out of it because my cable is better/faster but when there is an emergency/hurricane, etc. or I'm on vacation it is wonderful.

Think of it this way; a network is local, your home, the coffee shop/restaurant, etc., they each have their own network. To work "anywhere" you need phone service, 3G/4G. So, make sure if you want "anywhere" that you get 3G/4G, not just "network"/"internet". They are two different types of operations. If you have a cell phone, look; it probably has "both". You save money on your cell phone if you connect to a local network (your house) where you are already paying for the Internet; otherwise your phone data plan goes into effect and you pay through your phone. The phone is just a little computer these days, that happens to make phone calls