While the manufacturers hydration pack is exactly what 95% of users will need, sometimes a bit of variety is needed. Did you know that you can mix and match hydration bladders with other packs and accessories for a fit more tailored to your individual needs?
Have you found yourself liking the pack and bladder of one type of manufacturer but found some areas to be desired, well you can look into fitting accessories of a different manufacturer onto the pack. If you like the bladder of one type of hydration pack, but like the pack of another then you can mix and match different bladders and packs.
For those on a budget this can allow for a pack with more room for dry goods and swapping out for the bladder that they more desire. This could make for a very affordable "three day" pack that allows for 3 liters of water to be carried with small accessories that allow for water purification. It's always better to bring one small item that allows for purification on the go so you can fill up from nearly any water source than to pack the water that you will need for the trip.