Backdashing is used to create space between you and the opponent while still blocking mids and highs. This is useful to escape their pressure, and to make them whiff so you can get a whiff punish. It's not flashy, but this is extremely safe and often very rewarding. The average whiff punish outdamages the average low, and backdashing doesn't risk being counter hit or punished. Additionally, an opponent chasing after you is easier to hit, and an opponent who is careful with their spacing to avoid a whiff punish has to attack you less.
That said, it's important not to backdash yourself to the wall. With your back to the wall, your opponent can control their spacing much more than you can control yours. And if you backdash relentlessly, you'll never bait any whiffs.
Also worth mentioning is that different characters have different backdash lengths.