In what the fact that $\boldsymbol 1_{T<\infty}X_T$ is $\mathcal F_T-$measurable where $T$ is a stopping time is a good thing?

Question

Let $(\Omega ,\mathcal F,\mathbb P)$ a probability space and $T$ a stopping time. Let $(\mathcal F_t)$ a filtration and $$\mathcal F_T=\{A\in \mathcal F\mid A\cap\{T\leq t\}\in \mathcal F_t, \forall t\geq 0\}.$$ Let $(X_t)$ a stochastic process. In my course it's written that $$\boldsymbol 1_{T<\infty}X_T$$ is $\mathcal F_T$ measurable. In what is this a good thing ? First, I have difficulties to really understand what is $\mathcal F_T$, but also, I don't understand in what the fact that $\boldsymbol 1_{T<\infty}X_T$ is $\mathcal F_T-$measurable can be helpful.