What is the diff between the Media transfer protocal and Picture Transfer Protocol After connecting my device for data transfer the device asks for mode of USB transfer MTP or PTP.

I googled it and found that MTP(Media transfer protocal ) and PTP(Picture Transfer Protocol) are some sort of protocols for data transfer but didn't get why is it been used as we can transfer data easily with the previous USB mode as an external storage device.

The new USB device modes is somewhat complex for me to understand as I am used to data transfer only as an external storage device but not any PTP or MTP.

Which mode is suitable for what kind of data transfer?
MTP is the better option in most cases, since the protocol is actually designed for this use.

PTP is offered as an alternative since Macs don't understand MTP by default (you need to install "Android File Transfer" on the Mac to make MTP work). This protocol was designed for PCs to access photos from digital cameras, so it can only access the photos on the phone, and it might trigger a photo management app on the PC instead of a file manager.

To learn more about why MTP was introduced, see my answer to Why is Android File Transfer necessary?