It is to completely encase the mattress, not only protecting it from stains and wear but also protecting you from dust mites and other allergens.
Just one glance at the lining and the way they encase the foot will make you want to slip on a pair and relax with the morning paper.
But if you encase it in an impenetrable shell, it won't do you any good, right?
If it doesn't completely encase the butter, then stretch the dough a bit.
To be an excellent transvestite, you have to be meticulous about details, so it's not enough to shave and condition your legs; you also need to encase them in sleek and sexy shemale stockings.
Sigismondo entrusted the execution of his plans to Leo Battista Alberti, who had to encase in a shell of classic architecture a 13th-century Franciscan church.