"Is it just the capital saving of not buying a winch? Must be an expensive way to get a glider in the sky"
Nope - some sites (such as Aboyne) are too short to operate a winch (which is cheaper to operate). You can also be towed to lift which maybe too far away to reach off a winch launch or you can be towed far higher than the usual 1000 - 1500ft winch launch - handy for spinning / aerobatics / getting to wave lift etc.
Also the aerotow queue is usually smaller than the winch one so faster launches are assured, many more reasons for sure.
A matter of personal requirements over the size of your wallet
PS - Tug pilots are happy when towing, winch drivers are just plain grumpy!