Good reasons for someone in any field of work to take on a student on internship or practicum include:
- the opportunity for leadership skills development, and for personal and career growth, perhaps setting yourself up for a management role over time
- fulfilling an obligation to your field to train new workers
- returning the favour someone did for you way back when (call it karma if you will)
- maybe learning some new trend or skill now being taught in your field, that you were not fully familiar with before
- a chance to consolidate your knowledge/share what you know (with a set of willing ears!)
- getting some free/cheap help with a project that your team could use a little extra assistance on.
In my current work, I haven't yet met an internship host who regretted taking on a student. Some hosts got more out of the experience than others, certainly, but no regrets about doing it. The potential for gains seem very much to outweigh the potential for loss, so I encourage you to take the opportunity to supervise a practicum/internship student when you can!