The reason is usually down to getting the most out of the job. You , see when a homeowner calls a TV aerial fitter to carry out an aerial installation to another room in their home, the customer will not know whether they will need a new aerial or if the aerial they have enough quality signal for it to be fitted with a passive outdoor splitter from which other coax cable can be run into other rooms of the house to feed more TV points. Even in cases whether the signal is only good enough for one point, it can still be split using an amplified splitter device.
On a recent TV aerial installation in Preston, I was asked to fit an aerial that would feed 4 rooms in the house, they were the living room or lounge, the kitchen dining room, and two bedrooms. The whole set up would be fed by only one TV aerial. This would then be connected to an amplified splitter that sat at the base of the aerial mast. The power to the amp is sent via one of the coax cables, the amo also has a 25 db gain, so it is ideal if the original signal is weak, yet has good quaity.