However you still need a signal!
Clearly with no signal it can't boost one from nothing, however, there are antenna having far better signal gain than the one in a mobile phone.
Gleaning and then boosting an otherwise too weak signal for a handset to a viable one is what these 4G boosters are created to achieve. There must be any number of locations where the signal is too minimal for a mobile phone handset, but a purpose made system pulls in enough. A lot will depend on the bandwidth one's needs requires.
I carry two different provider's SIMs in mobiles, this extends the number of places I get all the signal I need, but my bandwidth needs are minimal anyway.