"Send as" users are practice staff whose names will appear on a task notification sent to clients. This will usually be the practice alias or the partner's or manager's name.
You might want to select a different user to the task creator if the person who sends the task isn’t the person that the client would usually deal with.
For example, Anna may be the partner who deals with Michelle, Clearwater’s business owner, but James might be the one who sends Michelle her tasks. If you want Anna to appear as the “From” name on emails the client receives and the task creator in the client’s portal, you would set Anna as the default “send as” user.