As with many things in life, for every original spare part or accessory there is somebody making a similar generic one. Whether it is printer cartridges, quality car batteries or dishwasher spare parts, there is always someone ready to sell you non original items.
Don't get me wrong, some of them are excellent quality and do exactly what they should; but there are very low quality items made in factories or sweat shops with low quality control. The problem is that it is difficult to tell one from the other and often you have no way of knowing that your non-original item is a quality product.
Sometimes too it can affect the performance of the thing it is being used for or even damage it, and many manufacturers state in their warranty terms and conditions that the item is only covered if original spare parts are used.
On the other hand, manufacturers are naturally trying to protect their market share so they will do everything in their power to stop people from moving to non-originals.
One trick too, is that some disreputable companies will try to pass off non-original products as original, so you have to be very careful not to be taken in in this case. The key is, always buy from a reputable source that you know and trust, for peace of mind.