All vegans and some animal-rights activists and environmentalists try to avoid purchasing products made with leather. Cows do a lot of burping, emitting tons of methane into the atmosphere, and methane accounts for 18% of greenhouse gases, second only to CO2, so the mass breeding cows, particularly to be slaughtered, is clearly bad for the environment, without me even getting into the environmental problems with trying to feed so many cows.
The problem with avoiding leather, besides the fact that it can be difficult in some situations, is that its primary alternative is vinyl, also known as polyvinyl chloride, or PVC (though vinyl comes in other forms). The production and distribution of vinyl is just plain nasty for the biosphere. Thankfully there are some alternatives, but they're catching on somewhat slowly. The easiest thing you can do to start ridding your home of PVC is to buy a non-PVC shower curtain. If you have vinyl flooring or siding that will obviously be more difficult and more expensive to replace, but it may be worth it for your family's health in the long run. And by not buying PVC products in the future (and potentially forcing producers to change to EVA), you may be helping the health of people across the globe.