To buy votes.  Nothing more, nothing less.

That is why the Conservatives introduced the Child Fitness Tax Credit and the Volunteer Firefighter tax credit and that is why the Liberals introduced the Teachers supplies tax credit and re-introduced the labour sponsored funds credit.

They are counter-productive ideas that are popular with a niche group of voters but sufficiently cheap that most people won't vote against them.  Politicians trade-off the votes they will gain from the niche group as exceeding the votes they will lose by implementing such a measure.