RTW suits used to come with two pairs of pants up until the 1960's or so.
There are a couple of reasons:
a) Men wore suits for both work and leisure activities (going to the ballpark, shopping, going to the movies); and
b) Men and women had fewer clothes, and the clothes that they had were intended to last. There was no concept of "disposable" clothing.
I used to get two trousers, but I no longer do. I found that I have enough suits, and it is unlikely that I shall prematurely wear-out a trouser on a seasonal suit, such as a summer suit.
In sum, if your suit wardrobe is limited, and if you are going to wear a suit frequently (e.g., once a week), then an extra trouser can be useful.