What is the motivation of using the Enhanced Fujita Scale in place of the Fujita Scale?
Both the Fujita (F) and Enhanced Fujita (EF) scales measure damage and then use damage as a proxy for wind speed.  Wind speed at the low levels is not directly measured unless the tornado is either very close to a doppler radar site or there happens to be a mobile radar sampling it.  This provides very few tornadoes with directly sampled wind speed and is the exception rather than the rule.

Where the F and EF scales differ primarly is that in the EF scale construction methods may be considered, not just the damage observed.

Damage is used as a proxy for windspeed by examining damage, construction techniques and methods (EF scale only), materials, and other factors and then determining what threshold wind speed could cause that damage.  This means that identical damage can result in different windspeed estimates.  

For example, a house that is completely destroyed and leaves behind a clean concrete slab is generally considered EF-5 damage, but some houses with poor construction have been found to not be physically tied to the foundation.  A particular case in northern GA, USA had tornado winds blow down a garage door and then lift the house directly off the slab and take it away.  This also leaves a clean concrete slab, but the construction methods will cause a lower wind estimate and a weaker tornado determination because less wind was required to cause the damage.

The only other difference between the F and EF scales are that some of the wind speed estimates have been lowered in the EF scale and more descriptive damage indicators are used for wind speed estimates to help standardize those estimates.

In summary, the EF scale differs from the F scale:


EF scale can consider construction methods
EF scale has more descriptive damage indicators used to estimate wind speed
EF scale has reduced wind speeds in each category to better align with engineering estimates of damage