Eggshells are good for plants and provide the plants some needed calcium and other nutrients.
When using eggshells for helping plants grow and thrive you should crush the eggshells up into tiny pieces so that the eggshells will break down and decompose easier.
Then put the eggshells into the soil around the plants or you can also mix the eggshells crushed up into your compost pile and the eggshells will add calcium and other nutrients to your compost.
Or you can also add the eggshells to potting soil to make the potting soil much better at helping your plants grow and thrive.
I use eggshells all the time in my garden and in my plants indoors and outdoors as well and it really does help the plants.
I always crush the eggshells up first though.