Why we use if, else if instead of multiple if block if the body is a return statement

if-elseif-else statements stop doing comparisons as soon as it finds one that's true. if-if-if does every comparison. The first is more efficient.

Edit: It's been pointed out in comments that you do a return within each if block. In these cases, or in cases where control will leave the method (exceptions), there is no difference between doing multiple if statements and doing if-elseif-else statements.

However, it's best practice to use if-elseif-else anyhow. Suppose you change your code such that you don't do a return in every if block. Then, to remain efficient, you'd also have to change to an if-elseif-else idiom. Having it be if-elseif-else from the beginning saves you edits in the future, and is clearer to people reading your code (witness the misinterpretation I just gave you by doing a skim-over of your code!).

What about the case where b1 == b2? (And if a == b1 and a == b2?)

When that happens, generally speaking, the following two chunks of code will very likely have different behavior:

if (a == b1) {
   /* do stuff here, and break out of the test */
} 
else if (a == b2) {
   /* this block is never reached */
} 

and:

if (a == b1) {
   /* do stuff here */
}
if (a == b2) {
   /* do this stuff, as well */
}

If you want to clearly delineate functionality for the different cases, use if-else or switch-case to make one test.

If you want different functionality for multiple cases, then use multiple if blocks as separate tests.

It's not a question of "best practices" so much as defining whether you have one test or multiple tests.