In industries that are somewhat mature, there are a little breakthrough inventions. Incremental improvements are the most new patents. This inventions won’t put your competitors out of the business, but if there are small improvements that is enough, it would add up to some advantage that is competitive. There are similar patents that your competitors have built. Imagine that there are thousands of these out there. How to determine whether you are infringing inadvertently on the patent of others, or whether they are doing it on yours? You would need to employ a full time army of engineers and lawyers just to keep up, studying patents and reverse engineering competitive products. No one would want to spend their money unproductively or, worse yet, spend tens of millions of dollars on infringement suits that may possibly end in a stalemate.
So there will be a détente instead. Like arsenals that are nuclear, patent portfolios are something you never use but you have it. Some people would call these patents defensive, by which they would mean that they will not be the first one to sue, but they will still maintain an arsenal with which they could countersue.
Keeping your portfolio up to date is one effect of this state of affairs, to maintain the power it has. Needing patents that are good deterrents is another odd effect, i.e. that would be applicable to the products of your competitors. So there are companies that patent ideas they are never going to use. This is considered purely defensive in nature.