Ideas are pretty worthless inside your head. It's hard to expand on them and spread focus on multiple aspects of your idea. If your idea is in your head, who will benefit from it? Most likely not even you.
Writing your ideas out will help you expand on them and make them better. Or maybe even great. It could help you attain your goal or help others attain theirs.
It is only the starting point when you write or draw them out but starting is much better then not.
I use a notebook and "notes" on my phone. Where do you collect your gems?