Two main reasons really: quality & cost. It can also be fun, therapeutic & give you a sense of satisfaction.

If you've made the financial commitment to purchase your own skis or board then you really should be doing your own tuning, not just for the cost savings (which can be considerable) but also for the greatly improved performance benefits - which after all is the reason you bought your own gear in the first place!

I'm not against shop tuning, in fact I offer a commercial ski tuning service myself. What I am against from a quality point of view is machine tuning which is basically what the vast majority of shops offer, as opposed to a hand tuning service that I offer & as performed by DIY tuners.

Now certain tuning operations, like stone grinding ski bases, have to be done on a machine & these are best left to a competent machine operator. However far superior results are achievable by hand tuning the ski edges & bases than can be had using a machine - without the additional wear caused by the machine operation. After all, the World Cup ski tech's hand tune their athlete's skis, they don't put them through a machine!

Ten reasons why to DIY tune:
Shop machine tuning grinds off excessive metal off the ski's edges which greatly reduces edge life. Regular hand tuning using mainly diamond stones removes the minimum amount of material to maximise ski edge life.

• Shop machine tuning grinds edges to the machine's pre-set angles. Hand tuning means that edge work can be done to the specific ski’s factory angles, or to any angles specified by the user, to give the best possible performance.
• Shop machine tuning cuts both the base & side edge angles which leads to an excessive base edge angle, increases the frequency that a base grind is required & reduces ski life. Hand tuning the side edge angle only (after initially setting the base edge angle if necessary) maximises control, the duration between base grinds & ski life.
• Shop tuners do not polish the edges after machine tuning. Hand polishing the edges after sharpening reduces friction & subsequent wear to the edges which maximises performance, edge strength & edge life.
• Shop tuners usually detune the tips & tail edges after tuning which reduces the performance of modern carving skis. Not detuning the tips & tails gives better turn initiation on carvers. If you do want to detune then a DIY'er can detune just slightly & then test etc to ensure the desired result.
• Shops perform unnecessary base grinds to remove base damage which removes all the impregnated wax, greatly reduces ski life & increases the cost as the edges then have to be completely re-cut. A hand tuner can repair base damage only as necessary, & then in a localised area. This gives the best repair adhesion, retains the absorbed wax in the base, negates the need for unnecessary edge work & minimises the cost.
• Shop tuners apply new wax on top of old which seals in the dirt & grime in the base which reduces future wax absorption, increases base wear & reduces base life. A hand tuner performs a hot wax/scrape to fully clean the base prior to waxing to maximise wax absorption, wax longevity, wax performance & base protection.
• Shop waxing machines remove the excess wax immediately after application which gives little time for the wax to be absorbed in to the base, giving minimum effect. A DIY'er can let the wax coat fully cool for the wax manufacturers recommended period before scraping, thus maximising wax absorption, wax longevity, wax performance & base protection.
• Shops use the cheapest wax possible which gives inferior performance. A DIY'er can use either specific waxes or the best universal temperature wax available (like Dominator Zoom) to give the best possible glide, control & speed in the widest range of snow conditions. A DIY'er can use base prep waxes (like Dominator Base Renew) & add additional layers of wax to maximise base protection & maximise snow time between waxings.
• Shop tuning if you’re a regular skier or a multi ski-owning family is going to get very expensive!