As BLLG Ultraversal has pointed out, you can overwhelm a Jedi if several people are firing at him at the same time.

But even if you're alone, it helps you. You might not kill a Jedi with firing at it with a standard blaster. However, you slow him down as he has to concentrate on deflecting the shots not to be killed. Otherwise, may simply can run towards you and use the force to speed them up. If you shoot at them, they (at least partially) need the Force to deflect.

Furthermore, there is a small chance that you indeed hit the Jedi. That's still better than just standing there and waiting for your death.

As stated in this answer, there are tactics against Jedi:

The real reason Slugs > Blasters against Jedi is really more straightforward than you'd think.

• Blaster bolts can be reflected, slugs will be burned up by the blade. This removes a significant counterattack from the Jedi.

• Slug-throwing weapons have a higher rate of fire. Jedi might seem impervious in the movies, but anyone can make a mistake. More projectiles = greater odds. Additionally, as others have pointed out, the higher rate of fire means more difficulty deflecting them.

• Slugs probably travel much faster than blaster bolts. While there is some controversy about the exact speeds of blaster bolts, based on a visual summary of the films it is clear that they travel much slower than even the slowest bullets on earth do. Speed counts against Jedi.

• Slugs are more difficult to see. While perhaps not always a total win against Jedi, less visibility is always an advantage.

Also, see this answer that projectile melt and pass through lightsabers. Therefore, firing at them is a valid self-defense option.

Humans have the fight or flight response to a stressful event (and I'd count a Jedi running towards you with an activated lightsaber as a stressful event). So, basically, you either run away or attack (fire your weapons) in hope to survive. They might be counscious that they're behaviour is more or less useless but their instinct / survival mode goes into "fire everything" mode.

As BLLG Ultraversal has pointed out, you can overwhelm a Jedi if several people are firing at him at the same time.

But even if you're alone, it helps you. You might not kill a Jedi with firing at it with a standard blaster. However, you slow him down as he has to concentrate on deflecting the shots not to be killed. Otherwise, they may simply run towards you and use the force to speed them up. If you shoot at them, they (at least partially) need the Force to deflect.

Furthermore, there is a small chance that you indeed hit the Jedi. That's still better than just standing there and waiting for your death.

As stated in this answer, there are tactics against Jedi:

The real reason Slugs > Blasters against Jedi is really more straightforward than you'd think.

• Blaster bolts can be reflected, slugs will be burned up by the blade. This removes a significant counterattack from the Jedi.

• Slug-throwing weapons have a higher rate of fire. Jedi might seem impervious in the movies, but anyone can make a mistake. More projectiles = greater odds. Additionally, as others have pointed out, the higher rate of fire means more difficulty deflecting them.

• Slugs probably travel much faster than blaster bolts. While there is some controversy about the exact speeds of blaster bolts, based on a visual summary of the films it is clear that they travel much slower than even the slowest bullets on earth do. Speed counts against Jedi.

• Slugs are more difficult to see. While perhaps not always a total win against Jedi, less visibility is always an advantage.

Also, see this answer that projectile melt and pass through lightsabers. Therefore, firing at them is a valid self-defense option.

Humans have the fight or flight response to a stressful event (and I'd count a Jedi running towards you with an activated lightsaber as a stressful event). So, basically, you either run away or attack (fire your weapons) in hope to survive. They might be counscious that their behaviour is more or less useless but their instinct / survival mode goes into "fire everything" mode.