It’s not that you have to replace your flat tappet hydraulic lifters. It’s just a good idea. You see, flat tappet lifters are not totally flat where the cam acts on the lifter. The seemingly flat area is actually slightly domed. You can’t really see the dome, but it’s there. The shape allows the lifter to ride on the surface a little smoother. If your existing lifters are worn at all, you’ll get slightly different lift out of every lifter and cam lobe. When building a new engine, you should try for consistency in all things. All parts are toleranced the same. Plus, cam and lifter kits usually include the cam and lifters as well as other pieces to install the cam, depending on the kit. Since lifters are so cheap when you buy in kit form, it’s just a smart move.