The point of TLC is to provide a preliminary check on the purity of the product, and to monitor reaction progress. Usually when you run a TLC plate, you also make a separate spot for your starting materials, and then if available, a pure product. You run this TLC plate, and you can see where the spots for your starting materials are, and then if you see more than one spot in your product column, you know it is not pure. You can compare the impurity spots to the starting materials to see if the reaction did not fully complete.
And TLC can provide semi-quantitative information as well! You can analyze reaction mixtures and determine which products are major and minor sometimes. As an example, I've attached 3 pictures (as a .zip file, they were making this post huge). This is a reaction I did. P stands for product, BP is byproduct, and RM is reaction mixture. This reaction produced two compounds (P and BP). However, we thought that we could convert the BP to P by refluxing in water and methanol. You can see that TLC provided us a quick way to follow the progress of the conversion. The first plate has two spots for the RM that are roughly equal in intensity. The second plate, the bottom spot (the one we want), is now slightly darker than the top spot. By the third plate, you can see a clear intensity difference between the two spots. This indicated to us that this process had converted our BP to our P.
TLC is an great, quick tool for any organic chemist to have. There's even more that you can use TLC for. However, for most undergrad organic labs, the purpose of you doing TLC is practice and to follow reaction progress/check purity.