Function returns only one value.  when we assign the final 
value with variable that name should be the same name of 
the function name.

In the given example, som is the function it returns the 
sum of two values when we assign that value to variable[i.e 
same as function name(example: som = c)]

if you don't assign, function doesn't return any value
(example: z = c)

Example:

Function som(a, b)
c = a + b 
som = c '(instead of these two step we can write in single 
step like som = a + b)   
End Function

x = 10 + som(10, 20)
msgbox x

you can pass this value in any other function.

Example:
y = som(som(10, 20), x)

msgbox y