Phosphorous is primarily introduced into a saltwater tank through fish food. It can also be added inadvertently by using an incorrect water source. As reef-keeping becomes more popular, a rise in the occurrence of phosphate introduction into a saltwater tank is through second hand live rock. (If you’re unsure of what live rock to start off with, read this article.)
If you are keeping coral, a decrease in phosphates has been shown to cause coral bleaching.
As with nitrate, phosphate cannot be expected to disappear of it’s own accord. It will be your responsibility to ensure you have a means of reduction and ongoing control.