For loop brackets - C like syntax

When i am using the below syntax to iterate why two brackets are needed ?

for (( expr1; expr2; expr3 ))
do
 command1
 command2
 ..
done

and the below code doesn't work ? and throws error "syntax error near unexpected token `('"

for ( expr1; expr2; expr3 )
do
 command1
 command2
 ..
done