I'm guessing reg force? Regular or Reserve, the army teaches good work ethics such as teamwork and discipline, it pays pretty decently, in many cases you can learn a trade which is transferrable to the civilian world, and regardless of your job civilian employers look well on people with military service for the reasons mentioned above.
You can make a lot of new friendships, boost your self-esteem, go past what you believe your 'limits' are, and be proud that you are serving Canada, and defending the rights and freedoms of your family and friends from oppression.
As for good things the army does in operation-terms, apart from the various overseas missions, the military has been involved in many domestic operations in the last several years, aiding in domestic relief. Canadian soldiers augmented emergency services at the Red River floods, helped to restore Quebec and Ontario power during the 1998 Ice Storm, and more recently helped to fight the forest fires in British Columbia in 2003.
There are many other reasons, but I'm tired and will go sleep now :boring: