Why should a potential measurement be grounded through a reference electrode

First of all, let's look at the measurement method of electrode potential: the measured electrode and the reference electrode of the precise known value of the electrode potential constitute a battery, measuring the value of the battery electromotive force, the electrode potential of the measured electrode can be calculated.

That is, if the reference electrode is given the exact value of the electrode potential, the electrode potential of the measured electrode can be obtained by measuring the difference between the measured electrode and its potential, and then adding them together. So we need to know the electrode potential of the reference electrode. The reference electrode is grounded so that its potential is 0, so that it can be calculated easily. The measured potential difference is the electrode potential of the measured electrode.