Okay, there are couple reasons I started to make my daughter's clothes.
I absolutely dislike( hate is a strong word, I try to avoid using it) the gender biased clothes on the market.
When looking for baby clothes before my daughter was born, I found most of the children cloths are either blue cars, green dinosaurs or purple butterflies, pink kittens and etc. I thought we are better at 21st century, guess not. I was looking heavily on gender neutral and earth colours, and it was so hard to find those. So well, easier to find the fabric I like and make it to whatever I like.
Material
For small children it is harder to find their clothes in special high quality materials. As we live in Quebec, our winter can be tough sometime. My partner and I also wish to raise our kid closer to nature, and we take her and our dog out whenever it's possible and weather shouldn't be an issue (unless it drop to -30, I grow up in a place with +15 in coldest winter, so...).
I remember I brought her first snowsuit in Costco when she was 18 month, it was ridiculous bulky and it was so hard for her to move around let alone running. That's the time I started digging, and I found out that layering is VERY important and what kind of materials you should wear in layering piece. I used to think natural fabric (like cotton, bamboo) is the best for our body. However, I then learned that natural fabric absorb moisture but doesn't let them go. As a result, if you use a natural fabric as your base layer (the first layer you wear that touch your skin) it will absorb your body heat/ moisture and keep it in the fabric. When it absorb enough moisture it gets wet, and then you feel the cold. Layering is such an art (lol), I am still learning to be honest. If you are interested to know more, there is a very knowledgeable article at Discovery Fabric, you should check it out.

Can be made according to body measurement
Well, both my husband and my daughter are in the skinny/ tall body type. So my daughter outgrows the pants I brought her fast at length. So when I am making her trousers, I cut the pants longer with bigger hem which I can release to lengthen the pants later on. Use the clothe for longer time, waste less materials, and nicer to our environment.
Fun
Oh, I don't think I need to explain here. How fun it is to make your kid's own clothes! With matching headbands, hats, scrunchies and all the little stuffs. Oh almost forget the fun pom pom skirt I made my daughter for Christmas party last year. - Although the truth is, she didn't like it = = She was too young, I am gonna keep it for this year's Christmas!

-Iting