Tug pilots aren't expected to "work" for free...after all they're not paid and aren't usually employees, so (technically at least) they're not working. Many gliding clubs have members who're happy (keen!) to fly the tugs to keep their hand in anyway. There's no obligation for tug pilots to do what they do....gliding clubs don't go around PPL schools press-ganging wannabe airline pilots and forcing them to live in a caravan. What they do is to provide keen, enthusiastic, pilots with an opportunity to hour build at zero cost.
While tugging pilots often have the chance to fly in a variety of types, in varying conditions and have a chance to do interesting stuff like looking out of the window to see what's going on instead of concentrating on the needles and numbers on the gauges. This gives them a chance to extend their spacial awareness, develop mental models of what's going on around them (weather, traffic, zone and noise abatement wise) and build their ability to be flexible (quite annoying when a glider suddenly lands right where the tug pilot had chosen!). They get to do a high number of take-off and landings which gives them the opportunity to practice their skills (usually in front of an audience who're watching them too!). They also become an important part of the club and are valued/appreciated/slagged off something rotten (usually in good humour of course).
All great fun.