FR24 and Planefiner are fine if you're happy to put up with their blocking-lists and hidden aircraft. However many enthusiasts prefer to know exactly what's flying over them, or to monitor their local airfield, or just to contribute to the general level of information on the Internet. For which £60 or £100 isn't an inordinate expenditure.
In terms of most hobbies that's a bargain. One eyepiece for an astronomer's telescope costs more than that and it wouldn't even be a downpayment on a modern camera lens.