Difference between the following two very basic circuits
There is no functional difference in reality. However, your simulation software is stupid and needs to do calculations, so you need to tell it which node you want to represent zero and will become the reference to which every other node is measured (remember that voltage is a potential DIFFERENCE).

The circuit with a GND connection has designated that node as zero volts, he node with which all other nodes are measured with respect to.

Note that physically making that GND connection to something else in reality doesn't make your circuit operate any differently, but does make it so your circuit doesn't "float" if you are using a battery source or some other isolated source. It anchors your circuit to a known potential and not doing so can become an issue for things like noise, shielding, EMI, and safety (arcing for high voltage things).