For any product-based business knowing what stock they do (or don't) have is crucial to being able to complete your customer orders and effectively manage your business.
So why should you carry out a manual stock check?
If you have a warehouse then errors will sometimes happen, some stock will get checked in incorrectly or stock will be damaged and scrapped without being correctly recorded. There may also be the occasional instance of the incorrect item being sent. A manual stock check will allow you to confirm that what you think you have, is what you actually do have!
Review of old stock and lines with only one unit - these sometimes slip under our radar, but they are taking up space in your storage and if you have a third-party warehouse then they are costing you money. Understanding what old stock, you have or lines with just one unit left that you don't want to replenish can help you make decisions about sales.
Inspection of stock - by undertaking a full check you can evaluate if anything is damaged and cannot be sold or needs to be discounted.
Basically, knowing what you have and what you don't have helps you to make informed decisions about your business. Manual check allows you to know how accurate (or not) your systems are!