(PLEASE ANSWER) Why is it necessary to add an indicator to the flask during an acid-base titration?

An indicator is itself a weak acid or weak base but a special one. The colour of the acid form is different from the colour of the base form. So you can tell when the pH of your solution has changed from being one to the other by seeing the colour change from one to the other. The particular pH where that colour change occurs depends on the particular indicator you use. The most common indicator in UG Chem Labs is probably the one called Phenolphthalein when it has its proton attached, it is colourless like water but when the proton comes off it is pink. This change happens around pH=8–9. any solution with pH below that (acidic) will make the phenolphthalein hold onto its proton and be colourless. Any pH above that will cause it to lose its proton and become pink.

Explanation:

they both have a y in them

"the periodic table is arranged in order of atomic number."