There are many benefits to growing your own herbs and vegetables from seed. For starters, you will pay more for a punnet of lettuce with 6 plants in it than for a packet of 650 seeds. Never mind if you end up with more than you need. The excess seedlings can be swapped with other local gardeners for different seedlings or other items they might have excess of. Plus, many seeds will last for more than one season if stored correctly. Just make sure you keep them in a dry, dark place.
Understandably, you may feel more confident starting out with a seedling which is easier to monitor and will take less time before it can be used. If so, then let the plant eventually flower and bear seed. Not only will the flowers attract and feed beneficial insects, but those seeds it produces will be far more suitable to your garden conditions than any bought seed packet or punnet. This then guarantees you a continuous supply at no further cost. Harvest and store some of the seed but don’t worry if some escape you. Over time you will find them popping up all over your garden. That means you don’t even need to learn when to plant them – they will decide for themselves.