depends on what kinda specialization you are interested in, at least for me it’s mobile app development. I have been making apps since college, I was interested in Japanese language so I made a kanji learning app [0], then because I’m a bodybuilder, I made a workout log app [1], recently because I started reading hacker news, I made a Hacker News reader [2]. I learnt a lot from the process, from architecture, design pattern to code quality control. I would say learning is the key, usefulness is secondary.
[0] https://github.com/Livinglist/Manji [1] https://github.com/Livinglist/Dumbbell [2] https://github.com/Livinglist/Hacki