I will further expand on leonbloy's answer by emphasizing the role of change of variables for integrals, but this will be a self-contained answer.

Assume $X$ is a real-valued random variable (defined on $(\Omega, P, \mathcal F)$). Let's say $X$ is some very complicated random variable and you wish to calculate its expectation because maybe it is an exercise problem and you have to submit a solution. Let's say you don't see how to calculate its expectation until one day you get the feeling that it might be easy to calculate the expectation of $X$ restricted to the slice $Y = y$ where $Y$ is another (auxiliary) real-valued random variable (on $\Omega$) that you came up with, and where $y$ can be an arbitrary real number, and you guess the expectation along the slice is $h(y)$ (where $h$ is some explicitly written function, maybe you guess $h(y) = y^2$, or maybe $h(y) = y+1$, ...), and now you think you just need to calculate $\int_{-\infty}^{\infty} h(y) d\mu(y)$ (where $\mu$ is the probability distribution of $Y$) because your intuition says that the result of that calculation is exactly the value of $E(X)$. (the intuition comes from generalization of the intuition in André Nicolas's answer)

Let's say you successfully calculated $\mu$ and then also $\int_{-\infty}^{\infty} h(y) d\mu(y)$. Now it only remains to rigorously prove that $\int_{-\infty}^{\infty} h(y) d\mu(y)$ is actually equal to $E(X)$ and you immediately see a little problem: the expectation along a particular slice such as $Y=2$ may have no meaning at all because $Y=2$ may be a null event.

I must mention that if one finishes a course on measure theory, he/she will have passed through denial, anger, bargaining, depression and arrived at acceptance of the fact that he need to live with many measurable functions that are only a.e. well defined (as opposed to everywhere defined), and measurable functions that come from theorems that only guarantee uniqueness with respect to a.e. equal. Then you will get some intuition that the little problem has a way out: (but not in the naive way: see Borel-Kolmogorov paradox).

While the expression $E[X | Y=2]$ (conditional expectation of $X$ given the possibly null event $Y=2$) eludes you for now, the expression $E[X|Y]$ is fine. If lucky, you may be able to prove that $E[X|Y] = h(Y)$ holds almost everywhere. Proving that might require applying properties of conditional expectation several times until you arrive at the desired conclusion $E[X|Y] = h(Y)$. Sometimes you start with an informal argument for why you find the (meaningless) equation $E[X | Y = 2] = h(2)$ plausible and then you keep replacing steps in the informal argument into rigorous steps until you come up with a rigorous proof of the (not meaningless) equation $E[X|Y] = h(Y)$.

Let's say you manage to prove $E[X|Y] = h(Y)$. Now what remains? The tower property now says $E[X] = E[h(Y)]$, but the change of variables (for integral) says $E[h(Y)] = \int_{-\infty}^{\infty} h(y) d\mu(y)$ which you have already calculated. So you are done. You have shown that $E[X]$ is equal to your calculation result.

Remark 1:

Some people write $h(y)$ as $E[X | Y= y]$. It is okay to use expressions like $E[X | Y= y]$ after all, if you and the readers are aware of the gotchas.

Remark 2:

Also known as the law of total expectation.

Remark 3.

If $\mathcal G$ is a sub-sigma-algebra, then we still have $E[X] = E[E[X|\mathcal G]]$. If $\mathcal G$ is generated by finite or countably infinite family of random variables, you can still give similar interpretation. For example, if $\mathcal G$ is generated by two real-valued random variables $Y, Z$, then $E[X] = E[E[X|\mathcal G]]$ is just another way of saying $E[X] = \int h(y,z) d\mu(y,z)$ where $h: \mathbb R^2 \to \mathbb R$ is some measurable function satisfying $E[X| \mathcal G] = h(Y,Z)$ and $\mu$ is the probability distribution of the joint $(Y,Z)$.

The intuition.

The intuition for the tower property is that for example $E[X] = E[E[X| Y,Z]]$ (where $E[X| Y,Z]$ is just $E[X| (Y,Z)]$) is simply a more succinct way of saying $E[X] = \int h(y,z) d\mu(y,z)$.

Remark 4.

Convenience of using something like $E[X] = E[E[X|Y, Z]]$ rather than $E[X] = \int h(y,z) d\mu(y,z)$ is in that with the former you keep the setup in a single probability space $\Omega$ while the latter involves two probability spaces $(\Omega, P)$ and $(\mathbb R^2, \mu)$.