Concatenation with char variable in SQL Server doesn't work?

Question

 For example:-

 

declare @a char(4)

set @a='ab'
set @a=@a+'c'

print @a

 

--it gives the output 'ab' but if declare the @a as varchar like

 

declare @a varchar(4)
set @a='ab'
set @a=@a+'c'
print @a

 

--then  it gives the output 'abc'