These tell you where a forced line-break occurs, every time you've pressed the Enter key, that is. Knowing where these are becomes important when you add or delete a few characters to a line. You might wonder why, unless you can see that paragraph mark giving itself away. Neither of these characters will appear in printed text. They may distract you for a few days, appearing on your screen, but soon they will almost become invisible as you write, only appearing before your eyes when you look for them.
Some authors also turn on spaces.
Some agents and publishers hate it if you use two spaces after sentence ends. All agents and publishers allow single spaces there. So it makes sense to use single spaces exclusively. Here's an easy way to convert, but there's a slight danger. The key is to use Search and Replace intelligently.