And why Catholic churches don't always have them.

In the United States we see pews as a necessary and basic part of every Catholic church. However, pews are a rather recent invention and surprisingly didn't even originate in Catholicism.

For most of Church history, worshipers stood during the celebration of Mass. There did exist a few scattered benches for the elderly to sit on, but in general the nave of the church was entirely void of places to sit.

This made practical sense, especially when kneeling became a common posture of the laity. Additionally, in medieval churches the pulpit was typically placed in the middle of the church, apart from the sanctuary. This meant the laity had to physically walk over to the pulpit to listen to the priest's very brief homily.

There was so much movement during Mass that no one, including the priest, ever had a chance to sit down.

Pews were essentially non-existent until the Protestant Reformation. In most Protestant churches the emphasis during worship services was not the many liturgical movements, but the sermon given by a preacher. The interpretation of the Bible by the local pastor was a chief focus of Protestant liturgies and led to long discourses at the pulpit.

Pews were gradually introduced over time and were especially popular in English churches. On account of the expensive nature of pews, individuals and families would purchase pews and guard them with their lives. In some cases they even constructed "pew boxes" to protect them, locking them up so that nobody else could use them. Unfortunately there even arose various legal battles over pews as individuals regarded their seats as personal property.

Later on when churches could afford the installation of pews, they still relied on parishioners for additional income and began to "rent" pews. This practice was brought over to the United States from England...

https://aleteia.org/2017/07/23/this-...were-invented/