Some vegetable seedlings are too fragile to be sown outdoors.
Vegetable seeds that have to be sown indoors, can be sown in a number of different places or systems. the main thing is that the tender vegetable seedling is given the best chance to develop into transplanting size - ready for the garden.
Some vegetable seeds need more heat than others to germinate. Some hardier vegetables can be sown directly outdoors - others have to have a warmer start to life.
'Indoors' can be either a greenhouse (heated or not) a propagator case, a window sill, an airing cupboard, or simply in a pot in the porch! Cloches are also classed as 'indoors!'
Other than the tender nature of some plants, the other reason to start seeds off indoors, is to give the plant a longer growing season. otherwise, if you have to wait until it is warm enough outdoors, your crop yield will be reduced as some vegetable plants take time to mature.
The sooner in the year you can get the plants
to mature, the sooner you can start cropping. For example, if you
were to rely on tomato seed planting outdoors, you will be sowing in
late May to June, and not cropping until mid-September. Earlier
sowing indoors means that you can have those tomatoes for the summer