Why?
Rate limiting an API is often required to ensure that clients do not abuse the available resources and that the API is reliably available when multiple clients are requesting data concurrently. Buckets can be created based on various parameters of an incoming request (eg. Authorization, IP address) to configure how requests are grouped for limiting.

Rate limiting functionality is already available in some proxies (eg. Nginx, HAProxy). However, they often use in-memory stores that make rate-limiting when running multiple proxies (e.g. for load balancing) unpredictable. Configuration for these limits also gets complex since it includes many actions such as routing, request/response re-writing, and rate-limiting.

Sphinx is not...
Sphinx is not focused on preventing Denial of Service (DoS) attacks or requests from malicious clients. The goal is to expose rate limiting information to clients and enforce balanced use by API clients.

Sphinx is not a request forwarding service. Sphinx only allows for very simplistic forwarding to a single host per instance of the rate limiter. Any advanced routing or request handling should be handled by a real proxy (eg. Nginx, HAProxy).

Sphinx is not an HTTPS terminator. This keeps the burden of configuring SSL certificates and security outside of Sphinx. Ideally, there is real load balancing and HTTPS termination before a request hits Sphinx.