Why limit yourself to 5W when a full licence lets you use 400W?

QRP is the 'Q' code for operating with low power. The definition varies depending on the 
operation mode and what people prefer. Typically, QRP is 5W or less for CW and digital modes 
while it is 10W or less for SSB. 

Anyone can make a contact with 400W and any old antenna. It is like using a sledgehammer 
to hit a nail. QRP forces you to think about the whole system and ensure you are not loosing 
any of your precious RF power to mismatched impedances or other traps. It makes you look at 
atmospheric conditions to see whether you have a chance today. QRP can also make your life 
easier. Low power means smaller diameter cables and lighter equipment. There is no need for 
a heavy amplifier or for heavy heat sinks either. Your neighbours are less likely to suffer 
from interfence if you use low power as well. Happy neighbours means less hassle when you 
finally get that dream mast set up. Another advantage is the reduced keep out area around 
the antenna required for meeting EIRP limits.

There are issues with QRP. It can be frustrating when you are not able to make those 
contacts regardless of your skills. Yet, that can also be a positive as it makes getting 
that elusive DX contact far more satisfying. There is no shame in boosting power on those 
days where 5W just is not cutting it. Many QRP operators use a QRP rig attached to an 
amplifier for those sorts of days. 

In the end, it is about trying to use the power necessary to make the desired contact. It 
does not matter if you want to use higher power or not. Just so long as you play nicely and
 use what you need instead of swamping the spectrum for everyone else. We all need to play 
nice so everyone can enjoy the hobby.