Help please with 1.1.9

[Roberts_Clif]

I setup my 3D Print it starts at the Bed then Raises to whatever Layer Height is set and starts the Print.

Do not understand the purpose of all the G1 Code lines
G1 Z15.0 F{travel_speed} ;move the platform down 15mm

G1 F200 E3 ;extrude 3mm of feed stock

G1 F{travel_speed}

G1 E-1 F300 ;retract the filament a bit before lifting the nozzle, to release some of the pressure

G1 Z+0.5 E-5 X-20 Y-20 F{travel_speed} ;move Z up a bit and retract filament even more

I only raise the Z-Axis to leave a clearance then Extrude 4mm to ensure the filament is extruding.

My Startup G-Code looks like this
;Filament used: {filament_amount}m {filament_weight}g
;Filament cost: {filament_cost}
;M190 S{print_bed_temperature} ;Uncomment to add your own bed temperature line
;M109 S{print_temperature} ;Uncomment to add your own temperature line
G21 ;metric values
G90 ;absolute positioning
M82 ;set extruder to absolute mode
;M106 S127 ;set Fan half speed
M107 ;start with the fan off
G28 X0 Y0 ;move X/Y to min endstops
G28 Z0 ;move Z to min endstops
G1 Z15.0 F{travel_speed} ;move the platform down 15mm
;Put printing message on LCD screen
M117 My Print...
G28 X0 Y0
G1 E4.0 ;prime extruder
G92 E0 ;reset extruder length
M420 S1 ; Bed Leveling On
M220 S32.000000
;M420 V1 ; Print the leveling grid