A lady recently asked me, "I keep my outdoor cushions in the patio. There is a roof. When raining, there maybe some drizzle on the cushions... Do I need to buy waterproof throw pillow covers or would water resistant once be fine?"

Firstly, I feel this is a very sensible question! I think this question is important for all of us who have an outdoor seating area. Thank you for asking. 

One of the necessary elements for mould/mildew to make a happy living on your outdoor fabric made-ups is moisture. Without moisture in the fabric it is very hard for mould/mildew to take hold. A water resistant fabric absorbs some moisture. This moisture can also reach the insert of a cushion. So, even with a light drizzle, some moisture will be absorbed. Even without rain, if you leave the fabric outdoors overnight, humidity in the air can be absorbed in to the fabric and/or through to the insert.

Of course water-resistant fabrics will absorb less moisture than normal fabrics. However, waterproof fabrics would not absorb any moisture at all. 

This means, waterproof fabrics, due to the absence of moisture makes a very hostile environment for mould/mildew. (But on top of this great outdoor fabrics are also treated for mould/mildew resistance). This of course means, you have to spend less time maintaining them... and they will last you much longer. 

Of course I didn't mention the obvious point that water-resistant cushions and their inserts may be wet if caught in rain or on a very humid night. Resulting in you being unable to enjoy the seating area as needed and the task of getting the covers and inserts dry again.... A very good waterproof outdoor cushion can also stay  completely waterproof under rain. This means, you can simply be hassle-free. 

Please note, there are levels to waterproof nature of outdoor fabrics - I hope to explain this in another post.

Today, the market has the option of waterproof outdoor fabrics/fabric-made-ups readily available. And they are also competitively prices. So, I believe given the convenience in use / maintenance and durability, waterproof cushion covers are definitely a better choice. Even if you leave them under a roof. 

Some increasingly choose waterproof outdoor cushions for indoors as well! Given the ease of maintenance and hygiene in not absorbing dirt/sweat through to the insert. Especially found important when the household has kids. But something to be mindful here is the softness of the outdoor fabric, when choosing to use indoors.