What are the benefits using J-Link debugger over printf or UART output? Can you look inside registers in real time and even change them? I don't understand why someone would need J-Link especially considering it's price.

i like this video. im trying to implement it in my project but having some truble. do you have any experience using the nrf_connect_sdk?

The trend is not to use JTAG/SWD as a printf replacement. There are many reasons, the first is you would really want to reserve the JTAG/SWD connection used for development and debugging and not program status and console functions. Usually a UART is reserved for real-time reporting at both the bootloader (sometimes call console interface) level and the OS (sometimes called program monitor) level. The console function (traditionally called before C main) provides a two way link to the Bootloader (sometimes called the BIOS) with a command line interface (usually minimal features). The UART function is easily supported by using a UART to USB converter probe interface and the basic UART/USB driver shipped with Windows, Mac and Linux. Using a freeware terminal program application as the interface. So the basic field troubleshooting tool set is a $10.00 probe, a USB to Serial (Virtual COM Port) OS driver included with Windows and a freeware terminal program. (For field troubleshooting at the customer, sending them a J-Link is really not viable, for the obvious cost reasons, but also many times the customer will not know how to use the Segger GUI, but they will know how to use a command line interface used with a terminal program, and if not they can be easily told over the phone what to do. Many times the UART COM connection uses 4 pins, RxD, TxD, RTS, and CTS (Sometimes DTR, DSR in place of RTS and CTS, which are basic digital port I/O pins), with the important observation that the PC is the "Terminal" and the product board the "Device" or data set (like a MODEM), as in Data Set Ready (DSR). Upon reset if DTR is not asserted, meaning there is no computer connected, the serial port remains inactive and C main gets executed as normal. After boot while the application is running, if DTR is asserted, by connecting a computer, then the OS monitor interface is invoked. Here the monitor can be a full featured command line interface which is very useful in diagnosing bugs. You really want to keep the JTAG/SWD functionality separate from serial port diagnostics and debug. printf real-time debugging is not really used for well funded projects. The UART is used as part of the project to support specific features.