Yes, they do!

I'm on my 4th day of the challenge.

Here are my personal pros & cons so far:

PROs:

Time constraint

You only get 1 day to complete a task, which totals in several hours, if you work full-time. I believe I will be able to tackle real life cases more effectively in future + better time-management skill.

Practice

I design in Sketch and want to master this tool. Variety of tasks allows me to try different techniques.

Creativity & abstract problems

For me, the challenge is a balance between creativity & problem-solving. Since I'm not solving any real problems, I might just train my creativity skill. For example, today I received a task to design a Calculator and I thought why not make it a speech-based. I set my own constraints which allows me to be a client for myself (very challenging!)

Review of own work

I review my work the next day. Fresh mind gives me the ability to see what could have been improved/done better. I think that's a big plus to experience.

Feedback

Sometimes you might be lucky to receive constructive feedback (if it's not Dribbble)((just kidding)). Good for beginners who have problems with visual design basics.

CONs:

Pretty much agree with everything what Volkan pointed out.

I would put it this way.

Pixel pushing

I will be happy if some of my designs get implemented in future, for now they are just pictures.

P.S. you can follow my struggles at:

https://twitter.com/sergeyjech or https://instagram.com/sergeyjech