The IDE Is the web interface to your smartthings account. There is some information there which is not available through the app itself. (It is also used by developers who are adding custom code, but that’s not the only purpose.)
Also, as @Automated_House mentioned, it’s not that there’s one category of “smartapps“ that has different features than “automations.“ Technically smartapps are also automations. The terms have changed over time and it does get confusing.
The original terms were “smartapps,” which was any code that ran in the smartthings cloud.
SmartLighting was a smartapp written by SmartThings staff and was and remains unique in that it is distributed with the firmware for the hub and so is available to run locally in some regions, including the US and the UK.
“Automations“ was a new term introduced with the new V3 app in 2018, and at first was just a fancy name for “rules.“ You’ll see in the app that rules that you create through smartlighting, for example, as shown in the app as “smartLighting Automations.“
But overtime, the new rules engine introduced with the V3 app became more sophisticated, and The rules you create when you click on the + in the upper right corner of the app and select “automations“ still run in the smartthings cloud, but are now distinguished from “smartapps“ which you can add as custom code through the IDE.
It’s all pretty confusing, but the main thing is just to know that there isn’t some major official distinction between the two categories, if they even are two categories. They’re just different terms used at different times in the smartthings documentation, and not used very consistently at that.
But at present, the biggest distinction is that only rules created through the official smartlighting feature are eligible to run locally. And even then it depends on the exact details of that rule. For example, you cannot change the mode locally.
To be honest, if local operation is important to you, smartthings isn’t the right platform to begin with because it’s still mostly a cloud-based system. There are a number of competing home automation systems, including Hubitat and Homekit, which run primarily locally. So it’s certainly possible, but it’s just not how smartthings is designed.