I have a very large site with about 1000 pages. Performance is very good after setting up a cron job.
If you clear the cache, all pages need to be regenerated. Of course you can wait until your visitors have requested all your pages, but then they will experience a significantly slower page load. Cache Warmer can automatically generate cache files for pages that have Full Page Caching enabled.
Each time you clear the cache, a signal goes to the Cache Warmer it needs to do his work. To let it do its work there are two ways:
1. Run the Cache Warmer job manually, or
2. Run the Cache Warmer job automatically via a cron job.
I recommend option 2. You can set the crob job to run each minute. It will simply exit if no pages need to be rewarmed.