tl;dr Standard environment apps "scale nearly instantaneously to handle huge traffic spikes", "cost-effective for applications that have significant periods where they are not serving traffic", use reasonably limited standard API + proprietary API
More importantly standard scales down to 0 while flexible needs a minimum of 1 instance running at all times. So when just prototyping/developing or creating a small app only you or a few users will use it's very cost effective to use standard.