Why to use rams when you attack ?

To understand why we have to use rams when we attack will
take the following example:
- the attacker have 20.000 Imperians;
- the defender have 20.000 pretorians and roman wall at
 level 20;
- the players have the same populations
 (they do not have bonus for small population) +
 (no bonus from any hero).

The result is that the player who attack do not destroy
neither 50% from defense: 

Attacker 
Troops      0 0 20000 0 0 0 0 0 0 0
Casualties  0 0 20000 0 0 0 0 0 0 0
Defender    
Troops      0 20000 0 0 0 0 0 0 0 0
Casualties  0  9830 0 0 0 0 0 0 0 0 
We will add 200 rams to the attacker and we will see
a grow with 30% at the defense destroyed:
 
Attacker 
Troops      0 0 20000 0 0 0 200 0 0 0
Casualties  0 0 20000 0 0 0 200 0 0 0
Defender           
Troops      0 20000 0 0 0 0 0 0 0 0
Casualties  0 12685 0 0 0 0 0 0 0 0 
We will add another 300 rams and we see another grow with 80%
comparatively with first attack:
Attacker           
Troops      0 0 20000 0 0 0 500 0 0 0
Casualties  0 0 20000 0 0 0 500 0 0 0
Defender           
Troops      0 20000 0 0 0 0 0 0 0 0
Casualties  0 18615 0 0 0 0 0 0 0 0 
We will add another 200 rams to the attacker:
Attacker           
Troops      0 0 20000 0 0 0 700 0 0 0
Casualties  0 0 18816 0 0 0 659 0 0 0
Defender           
Troops      0 20000 0 0 0 0 0 0 0 0
Casualties  0 20000 0 0 0 0 0 0 0 0

We observe that the attacker will destroy all army and will
remain with army if he attacks with only 700 rams who are
be ready in only 5 days. A grow with over 100% at efficiency.
You will say that 700 rams eat a lot of crop and there could
be put other army in his place.
We will make the next simulation :
we change the 700 rams (2100 crop consuming) with 2100
Imperian (2100 crop consuming).

Attacker           
Troops      0 0 22100 0 0 0 0 0 0 0
Casualties  0 0 22100 0 0 0 0 0 0 0
Defender           
Troops      0 20000 0 0 0 0 0 0 0 0
Casualties  0 11284 0 0 0 0 0 0 0 0

The result is very weak comparatively with what we win
if we use rams.

Original source from http://www.travian0.com