Your natural lashes have a cycle of 30 - 60 days before they will naturally drop or shed off, and new lash will grow. When your natural lash shed, your eyelash extension will follow suit as the extensions are stick to your natural lashes.
This is where you might need a lash refill, as your extensions might have shed off due to naturally shedding of you natural lashes. Furthermore, your natural lashes are shedding at different times, thus you will encounter situation where gaps start appearing in your lash line.
This is when you need a lash refill to fill up the gaps.