What is the use of ISP and ICSP in Arduino?
ISP/ICSP has no role when utilizing a typical, properly functioning Arduino, as the program code compiled from sketches is instead downloaded via a bootloader (either serial, often behind an on-board USB-serial converter, or else directly USB).

The role of ISP in the Arduino ecosystem is mostly limited to:


Loading a bootloader into new chips which do not have one, or have somehow lost theirs (note however that you can buy pre-loaded ATmega328's from many sources at a small premium)
As an alternative program loading scheme when the tiny amount of program memory used by the bootloader must be re-claimed for a particularly large sketch
Programing alternate targets to which the Arduino libraries have been partially ported, especially resource-constrained ones where the bootloader takes up a larger fraction of the total memory, such as the the ATtiny25/45/85
Replacing or modifying the firmware of an on-board USB-serial micro such as the 16u2 on an Uno to perform a different function, such as emulate a keyboard or mouse.  However, this chip can also be programmed over its USB interface in DFU mode.
Programming other non-Arduino devices used in a project (though these often have a different ISP scheme than the ATmega series).


But a typical project using purchased board(s) will involve none of these.



SPI vs. ISP

As jfpoilpret points out, a very popular way of interfacing chips to a microcontroller is the Serial Peripheral Inteface, or SPI.  Like ISP, this is a synchronous (has a distinct clock line) serial interface with a master and a slave, a mode or enable pin, and a distinct data line in each direction.  Some Arduinos use MCUs like the ATmega328p which re-use their SPI pins for ISP (with the reset functioning as the mode pin).  

In effect, ISP is a sort of special "SPI-like" interface mode for programming an ATmega - often using the master-mode SPI engine of the programer to do the actual transfers.  But "ISP" applies only when programming an ATmega - the more generic case of communicating with a peripheral would be "SPI".  

It's my impression that most GSM modules use asynchronous serial interfaces (ie, utilizing a UART engine), but there may be some which use SPI or other synchronous schemes, at least as an option.