Want a fully autonomous UAV? Making your own drone lets you ......
A sense of self-achievement: If you want to feel like a designer and watch the drone take off the ground, then building a drone will give you a revered feeling of achievement.
Upgrading: If you build your own drone, each part can be replaced or changed with an improved one. Whereas, a purchased drone cannot be upgraded or replaced with something better.
Enriches learning: Making your own drone certainly would enrich your learning experience as you watch how things work and coordinate. You will gain tons of knowledge on assembling the parts, fixing them when damaged and then how to fly it.