Question: Sorry if this is a very basic question, but I am not a physicist, but this is something that has always intrigued me. 

I have read that the properties of a black hole are its mass, spin and angular momentum. And that lower/higher the mass, higher/lower its density and so on. Although singularities are described by mathematical models, it's hard (at least for me) to believe that a singularity actually exists in the middle of space in every black hole. So why is it that singularities are actually hypothesized? Can the extremely dense nature etc. of black holes not explain its properties with a finite volume?

Also, whenever black holes are visualized in animations etc., it's always an event horizon and the observer still keeps falling through what looks like empty space. What is the significance (and the location) of the mass of the black hole itself, then, ignoring the mass it accumulates/radiates throughout its lifetime?

Answer: 
The singularity is not a physically established fact but a prediction of a classical field theory (that's what general relativity is). Such predictions are usually seen as a sign that a theory breaks down in a certain regime or on a certain scale and that it has to be replaced with a better theory.

Unfortunately for us, no measurement has been made to date that contradicts general relativity, so we simply do not even have a slight hint of evidence of what to replace it with. We can speculate about what happens near the singularity predicted by general relativity and have done so plenty, but there is, at this time, simply nothing available that could tell us which of these speculative solutions is the correct one, or if we need something completely different that nobody has guessed, yet. 

As for beliefs... that's not how science works, so there is no need to garner any. Science starts with empirical evidence and then compares it with hypothetical explanations. The explanation that fits the evidence best is eventually elevated to the rank of a theory. As of now we simply do not have a good theory of what is "inside" a black hole. "We don't know" is therefor the correct and perfectly adequate scientific answer. We think we understand what general relativity predicts to be in there... and that's called a singularity (and it's not even a point like object if I understand some of these predictions correctly). I doubt that, figuratively speaking, more than a handful of physicists think that's the entire story, though.
The other answers have done a good job of covering the fact that we don't know if singularities actually physically exist, they might represent a breakdown of our theory, etc.

Here's another point which I think is worth making. Within the theoretical and mathematical framework of general relativity, singularities are *guaranteed* by the singularity theorems of Hawking and Penrose. Roughly, these theorems require the following things to hold:

-The spacetime you're working with satisfies certain energy conditions (we can use very weak ones to prove the singularity theorems, they're pretty physical and not very stringent).

-The spacetime satisfies certain global causality conditions (essentially guaranteeing no closed timelike curves allow time travel to the past).

-There exists a surface in the spacetime from which we could emit light and it could not escape to infinity (a formal way of testing that our spacetime contains a black hole).

If these things hold, the theorems guarantee that a singularity exists! So *within the context of general relativity*, singularities are inevitable. This was not obvious at first, and people worried that the high symmetry of our exact black hole models, and that maybe realistic black hole collapses might not have singularities. The singularity theorems well us that this is not an out. Very symmetric models do not explain away singularities, the conditions for the theorems to hold are very flexible.

Now, this guarantee requires that general relativity be a final theory which won't fail in the sort of regimes where singularities form. We think general relativity will probably have to be modified into a full theory of quantum gravity to make real predictions inside black holes, but general relativity taken literally requires singularities. It gives us reason to suspect that even if singularities don't survive in the full theory, they almost do in the sense that there will be extremely dense regions in the interiors of black holes.