One, you know your students WPM rate.  Gilmore and Gilmore (1968)  offer the following for representative reading rates: 
 
 

Oral Reading Rate:  Grade Level              WPM  1.8                                30-54  2.8                                66-104  3.8                                86-124  .  .  .  7.8                                122-155

Silent Reading Rate:  Grade Level                     WPM  2                                            86  3                                           116  4                                           155  .  .  .  9                                            252  12                                          251

Thus, if you are testing a fourth grader, and he/she reads at an oral rate of 42 WPM, you know that they need help increasing their rate (assuming you test them on a fourth-grade excerpt). 

Two, you know your students approximate independent-through frustrational level.  Thus, if your fourth grader frustrates at fourth-grade reading material, than you know they will struggle in class. 

Three, you know at what level of work to expect your child to read assigned work. 

Four, you can begin to detect pattern errors, by reviewing your recordings. 
 

The Informal Reading Inventory can be used in a variety of ways.  Remember, it is a tool designed to help you help your students.  It is NOT a precise diagnoses, but a good tool for gaining more insight into your student's reading, and in assigning work more appropriately.   

The author hopes this was helpful.