The other day, I performed a seam test for water leakage without seam sealing tape. I confirmed why most outdoor backpacks have been made with plain seam that sews together on the inside.
It is superior to welt seam in process cost and water pressure resistance. Both of the seam types didn’t prevent water leakage, but there was a clear difference in the leak time between them.

Welt seam has a firm and solid appearance with its flattened edge due to top stitch. I think it is more suitable for the backpack in business or city use.

The pressure of 200ml water I poured for the test was about 29.4 mmH2O, which is far from the standard of water pressure resistance for light rain, 300 mmH2O.
If you aim for waterproof function to your backpack, seam sealing will be necessary.

I hope this results will be helpful for your making or choosing of outdoor gear!

Fabric: X-pac VX21
Zipper: YKK, AquaGuard(Coil), 5CNT10CH
Silicone sealant: Konishi bond, bath bond Q, 04884

You can know details in my YouTube channel, “Backpack Researcher.” Please checkit out!