Grounding in circuit diagrams
The ground symbol:



means, "this is connected with wire to all the other things connected to other instances of this symbol". Since there is only one of these in your example schematic, it means nothing at all in this respect.

It also might mean, "If someone gives you a voltage without an explicit reference, assume it's relative to this thing".

So it serves two goals:


make the schematic more readable. There are a lot of things that connect here, and without this symbol we'd have a lot more lines in a big schematic, and you'd have to follow them very far to find they just go to the power source.
make discussion of the circuit shorter, since we can assume a voltage reference. Since voltages are relative, we always need a reference.


Note that discussing the circuit, and making it readable, really have nothing to do with how the circuit operates. The electricity in the circuit cares about the ground symbol as much as it cares about the identifiers by the parts, or the title block, or the border. That is, it doesn't care at all.

What happens if we stick the ground symbol between the resistors? The voltages become:


C = 50V
B = 0V
A = -25V


This doesn't change the operation of the circuit; just our discussion of it. We put the ground where it will make discussion convenient.