Apple try to prioritise in-store troubleshooting as it's much more cost effective for Apple. In-store software/firmware troubleshooting can be carried out to isolate the issue. In-store the technician will attempt to run in-depth diagnostics, verify the issue is not a user error, may attempt to reinstall Mac OS, and perform firmware troubleshooting via a Revive/DFU Restore, via another Mac, including for power issues. If unsuccessful the retail employee will depot the Mac which essentially is the same as a Mac mail-in with Apple Support over the phone. With Mac Mail-In/Depot it is sent to an authorised repair partner, determined by the model/issue, which will cost Apple a lot more, a flat rate, which Apple covers the cost of if covered by warranty or AppleCare. Therefore if it is an issue which can be resolved in-store, such as a software/firmware/usage/environmental issue, Apple will not have to pay a flat rate to the repair partner if it is resolved in store, otherwise if the store technician has verified the issue as hardware, the depot is required at that flat rate. A high percentage of referred cases to Apple Stores from Apple Support classified as hardware issues, can be resolve by software or firmware (with another Mac or equipment not accessible by the end-user) without actually requiring a hardware repair, therefore Apple can save hundreds by not sending to the repair partner which charges a flat rate fee for service no matter if hardware/software/firmware. It may also save the end-user time if the Mac isn't required to be sent out of country for two weeks and can be resolved in a day, in a store.
When I had an issue with my Mac, I was referred to an Apple Store, firmware was attempted first, which appeared to work, although it failed an in-store diagnostics, therefore it was a "depot repair/mail in-repair" from the Apple Store, as per the paperwork I received the flat rate charge was over £500 which was thankfully covered by my AppleCare and it was sent to a company in the Czech Republic from the Apple Store in the UK to perform this repair. Therefore I done my research and discovered how the process works.
All Apple Stores do have repair rooms and perform in-store repairs, although this is reversed for desktop Macs, which are not easily shipped, and Mac models classified as vintage, if parts are still available in the store.