A custom suit will have hand-stitched canvas lining. Canvas is flexible, hydrophillic (breathes), and most importantly!.. Over time, it takes the shape of your body. Meaning: the more you wear it, the better it fits you!
Ready-made suits typically use fusible (glue) interfacing that will not give, and shapes lifelessly compared to canvas lining. The fusible interfacing is synthetic and hydrophobic as well.
Canvas helps your suit last longer, and you’ll feel more comfortable, too. And Menswear is all about the comfort, right? :)