I also need to try out different needle sizes to decide which size will give me the fabric characteristics I want for a particular design.
There are other very good reasons why you should swatch. If the pattern uses stitch patterns or techniques you are unfamiliar with, wouldn't it be a good idea to practice first? A friend of mine showed me a beautiful cable cardigan she wanted to knit for her daughter. I knew she was an in- experienced knitter and I didn't want her to waste money on buying yarn for a garment she couldn't complete. So I advised her to get one ball of the yarn & knit a swatch of the cable pattern. The cable pattern was complicated & used 2 cable needles at the same time to work the cable. When I next saw my friend she told me she'd found the cable too difficult & had chosen an easier design & was relieved she hadn't bought all the yarn or started the knitting before discovering it was too advanced for her.
You could also use a swatch to practice any shaping techniques you are unfamiliar with.
So that's a few of my suggestions for why swatching is a worthwhile task. You can even keep your swatches & sew them together into a blanket. I donated a bag of my old swatches to a friend who does yarn bombing. She's always in need of knitted pieces she can use in her yarn bombing & I had a drawer full of swatches I wanted to clear out. I felt good about someone being able to use them.
When Em was younger she had a small teddy & several of my shawl swatches became Teddy's shawls. I also use them to explain how to achieve different shawl shapes when I teach.
So have I convinced you to take the time to swatch? Can you think of any reasons I've forgotten? Let me know in the comments.