Depending on the aperture that you are shooting with - say for instance, a 0.050" aperture for targets and a 0.093" (or 0.096" - depends on which rear peep you mount) aperture for hunting as examples. Some will use even larger apertures - 0.125" up to 0.150" if they are in the woods where critters with teeth and claws lurk.
For me, using a 0.050" aperture on targets, my rear peep reduced my groups by 1/2 compared to the groups I was getting with the Marlin semi-buckhorn factory rear. Peeps make it easier (for older eyes, in particular) to focus on the front blade - which is where the focus should be anyway, not the target. The eye will tend to center the front post naturally in the peep regardless of minor variances in cheek meld on the stock and so on, so it makes target acquisition pretty easy IMO. Other people may have other opinions, but short of scopes, all my iron sighted rifles (except one) have peeps on the receivers. To me, they are easier to use, quicker to come on target, and (at least, for me) roughly twice as accurate.
Doc Skinner makes a nice peep for the 1894, the lines blend really nice with the rest of the receiver, installation is easy, and the sight is pretty robust. Plus he makes different apertures for them. Check out his web site on the industry partners section.