Rubber is heat resistant so it would protect him from a fire as you see after the batplane crashed.

Get it was to bulk up Keaton, Burton was going through the trouble of distinguishing Bruce Wayne and Batman by having Bruce Wayne smaller and thinner than Batman. Drawback was it lacked flexibility, issue with the foam latex molding no matter how well you design the sculpt of the suit is that your pretending the material is something else, with the Affleck suit which still bulks you up it's like american football gear with the padding underneath fabric.

http://www.avclub.com/article/batman-forever-your-stereo-238785#comment-2759445056
Keaton was the only actor who made the rubber body suit make sense. I still don't like it, but it helps explain why no one would ever connect Bruce Wayne to Batmanthey aren't the same physical type.

In the comics he has bulletproof armor under his fabric bodysuit.

Also Rubber/latex is heat resistant so it would protect him from fire.

They had padding but I doubt padding under spandex would have looked that great either.
Why would it looked good back then?
They had no idea the rubber would restrict his movements, it was really thick, they tried to make it flexible in later films but the rubber could easily rip. TDK achieved the head movement by dividing the head cowl from the neck and the suit broken into multiple pieces of armor.
One problem about the body armor it gives away to his enemies he needs body armor, in the comics he has armor hidden under his suit.

They had padding but I doubt padding under spandex would have looked that great either. But consider that the suit was meant to be bullet proof/armor. Maybe they thought rubber would look more substantial. It seems like they stuck with that philosophy not only throughout the 90s series, but the Nolan series as well.

Could've been that. Don't know if they had padding back then to put under costumes to bulk someone up.

I may be wrong, but I'm not sure the materials used to make the BvS suit and other modern superhero suits were available in 1989. It seemed to me like the only choices were foam rubber or spandex.

Cloth version of the Keaton suit.
http://orig06.deviantart.net/b195/f/2015/223/9/e/batman__ben_affleck__black_batsuit_by_alexbadass-d8rp3q2.png
http://orig15.deviantart.net/3c05/f/2015/121/8/0/ben_affleck_batman_by_soyelmejor999-d8rqxhv.png

It can be heat resistant if it's foam latex in the context of the film.

Actually from the beginning Burton just felt he was black but yes black can cover the shortcomings.
In the Dark Knight Trilogy in Begins they were afraid of making it as thin as possible it would tear, in the sequels the suit was broken into 100 pieces of armor so he could move.
Now with DCCU they have a more flexible cowl and the rest of the suit is fabric stretched over padding.

they didn't know it was gonna be restrictive until he was wearing it.
I think that once they realized it turned out to be restrictive and hard to move around in, they made some trimming adjustments to it(around the limbs at least). If anything, the studio and costume designer probably decided to come up with a cover story that Burton wanted the costume to be mostly black instead of a traditional dark blue and grey color scheme when in reality they rather it would be black so as to hide the shortcomings of the much needed foam rubber molded batsuit.
Because ever since Batman was being optioned for a big budget motion picture, the idea was still there for him to have the traditional blue and grey colors on his costume.
They tried to make the rubber more flexible in later films but could easily tear.
I had no idea about that. Thanks. I'm kind of surprised it could easily tear if the rubber would be made more flexible. Was this also just as much of an issue for the Dark Knight trilogy? I thought by 2005, things would've improved with the costume by then.

The cowl looks so cheap when he tears it off in Batman Returns.
Originality needs a reboot.

Yes it was just to build up Keaton, they didn't know it was gonna be restrictive until he was wearing it. They tried to make the rubber more flexible in later films but could easily tear.

Wasn't it because that material was more useful for having the muscles sculpted out since Michael Keaton wasn't very muscle bound or at a peak athletic build? I think it was more of a problem for him when it came to the mask that he could barely move and turn it around. It was hard to hear anything and I think that's where it was too hot for him to wear.