Caching services gives you a faster website
Elasticache, Varnish, Memcache and Redis are so called in-memory cache services. Simply put, it means your website is being served out of memory (RAM) in stead of a hard disc.
With RAM information can be found much quicker than from a hard disc. Since a lot of visitors to your website all view the same content, it can easily be sotred in the cache. The cache itself figures at certain set intervals if the content has been renewed or not. If the content has been updated, the cache will also update.
The advantages are that your website is being served faster and visitors will have a more pleasent experience. This will also benefit your Google position. A second big advantage is that it helps with big peaks in traffic. Especially for high traffic websites this is very interesting. But also for smaller websites who have temporarily gained in popularity. Do you have a webshop for instance, that peaks a lot before the christmas sales? With the Varnish caching service your sites capacity can grow up to 10.000 at the same time. Usually a normal website gets slower with 100 concurrent visitors. The differences in capacity are really that big. See the test results below.
10.000 concurrent visitors and a page load of less than 2 seconds
The same website as above, but without Varnish. At a 100 concurrent users it already crashes