Reactivity information tells you how stable the chemical is under normal conditions of use and whether it reacts with other types of chemicals.  The chemicals you work with in the office should be stable under everyday conditions of use.  That is they are not going to ignite or explode in your face.  However you do need to be careful not to mix some chemicals together because they may react, producing a fire or toxic gases.  A common example is mixing chlorine bleach and ammonia which results in a reaction producing toxic and irritating gases.  In a fire, burning chemicals can produce harmful gases.  The most common is carbon monoxide, which is produced whenever you burn something.  However, chemicals can produce other toxic gases which are listed in the MSDSs.  That is why it is important to evacuate during a fire.