Situations where "Without Loss of Generality" expressions can be used.

Yes you are right. WLOG can be used when enforcing a particular condition can be made without really changing the nature of the problem because there is a way to transform the general case in a more particular one without impairing the solution.

Another example: to establish the formula for the radius of the circle circumscribed to three points $p_0,p_1,p_2$, we can admit WLOG that $p_0$ is the origin. Because if it is not, we can translate all three points by $-p_0$ without changing the value of the radius, and solve for the points $0,p'_1:=p_1-p_0,p'_2:=p_2-p_0$.

Another:

For the equation

$$a\cos\theta+b\sin\theta=c$$ we admit WLOG that $a^2+b^2=1$.

Because we have the equivalent equation $$\frac a{\sqrt{a^2+b^2}}\cos\theta+\frac b{\sqrt{a^2+b^2}}\sin\theta=\frac c{\sqrt{a^2+b^2}}.$$

Your understanding is almost right but I wont say it is used only in symmetric cases. The expression is useful because one can define the proof using a concrete example and any such example must be broad enough to not deviate from the general sense of the proof. So the pigenhole principle example given in Wikipedia article elaborates it quite intuitively:

If three objects are each painted either red or blue, then there must be at least two objects of the same color.

A proof:

Assume, without loss of generality, that the first object is red. If either of the other two objects is red, then we are finished; if not, then the other two objects must both be blue and we are still finished.

In addition to other answers, WLOG might be used to shorten proof or also to work with concrete notations and objetcs, instead of arbitrary ones.

For example, working in the space of continuous functions over $[a,b]$, $a,b \in \mathbb{R}$, is the same as working over the space of continuous functions over $[0,1]$, since we are able to from one to the other by the affine transformation $f(t) = a+(b-a)t$. Thus, every problem lying in the first, general space for any real bounds $a,b$, can be considered WLOG in the second one, which is fixed and easier to represent.