Because a the larger the cache, the more likely the the data you want to access is in that cache. For the cost of the 32GB Optane drive(roughly $80), you can get a 240GB SATA SSD. You can partition out 60GB to use as a cache, and that leave 180GB to install your OS and a few commonly used programs. Than you have a 60GB cache you can use to accelerate your HDD. This method of cache acceleration is pretty much just as fast as using an Optane drive with an HDD.
What are you using to defrag then? Because most defrag programs won't run on an SSD. They instead just do a trim function, which is normally super fast anyway. Using a normal SSD to accelerate a HDD need a reboot too to set up. It goes through basically the same setup process.
No, I'm talking about both read and write caching. When accelerating an HDD, there is no noticeable difference between using and Optane drive and 60GB of a standard SSD. Yes, there is a measurable difference, but not a noticeable one. You won't notice that Windows is booting 1 second faster(literally, that is the difference, 1 second), you won't notice that Chrome opens in 2.5 seconds instead of 3 seconds. You just won't.
And if you are trying to accelerate a normal SSD with an Optane SSD, you are wasting your money. Because, again, the difference isn't noticeable.