Basically in almost any scenario where I need to do some multitasking. Few examples:
Example 1
- I am listening to one song in Spotify
- I'm also browsing the web
- After a minute or two, I notice, no music is playing
- Go back to Spotify: boots from start (e.g. Spotify logo for a few seconds before continuing where I left off).
(and, if I've got some bad luck

- After selecting a song, go back to Safari... only to find out the webpage refreshes.
Example 2
- I'm writing something on a website using Safari
- I need to look something up in another app
- I go back to Safari, webpage refreshes and I lost my progress
Example 3
- I'm using Facebook
- Now I'm going to use some Twitter
- Some Whatsapp
- Some Snapchat
- Back to Facebook: it's got to reload again (Facebook logo for a few seconds before I can use the app again) *damn it*
- Back to Twitter: it's got to reload again *damn it*
- etc.
These are just a few examples. It's not something that is 'immediate'. E.g. I can open Facebook, than Whatsapp, than Snapchat and than Facebook again in just a few seconds and multitasking is doing just fine. But if I use all these apps for a few minutes before switching back, they all have to load from scratch.
Apparently, it's not an issue for everyone, but for me it is. And what is even worse, in my opinion, is that the now nearly 3 year old iPhone 5 does a better job at multitasking.