1. No weeds
2. You do not need to work the soil
3. No Deadheading
4. No Bugs
5. No need to worry about watering
6. The soil conditions are always perfect, no matter if you are planting on cobblestones or not.
7. Your fingernails never get dirty let alone broken when planting
8. You never break a sweat when gardening, even if you plant an entire sim.
I'm sure others could add to this list quite easily. However, there's one major point that gardening in SL and RL both share...the satisfaction of beautifying your surroundings and the pleasure of a job well done.