What is the use of SD card in MacBook pro?

If you mean the SD card reader, it is very useful.

Generally, there are two usages.

One is to insert a large-capacity SD card as a storage medium.

For example, some people only have 128G/256G hard disk, so they choose to buy a large-capacity SD card to save documents.

The second is a simple card reader. For photographers or photographers, their cameras are basically plugged with SD cards, so it is very convenient for computers to bring their own card readers.