In Jain tradition, a strainer is used to filter water so that very small living beings or impurities are not consumed, in the spirit of ahimsa.

You should drink the water after straining because:

• it helps reduce harm to subtle living beings in the water,
• it makes the water more suitable for Jain consumption,
• it supports careful, mindful living and non-violence.

If you mean a specific Jain practice, the general principle is: strain water first, then drink it to follow ahimsa as carefully as possible.