im my own terms so bear with me lol......a dial indicator is a "dial" that has a tip protruding (google it for a pic) from it that sits on the surface that you want to measure, as you then roatate the shaft/bearing/valve/crank shaft ect it moves the needle on the dial showing error in alignment (all the way down to 0.001mm) to show or wether the piece is within specified tolerance or out of alignment. something such as a turbo will have very fine allowable factory spec tolerance with its shaft movement and also compressor and exhaust turbine blades. what that is i dont know but im sure mr google will tell you.
basically movement from your hand is defiantly not an accurate way to measure something like end float, or any engine specification for that matter. thats why we (techos) use dial indicators, micrometers and boroscopes when checking wear on machinery components.
some measurements can be bloody ridiculously small.
hope that helps, or at least makes sense lol. im not the best explainer.