Computer modeling of solvent-immersed polymeric brushes with anisotropic attraction and polyelectrolyte brushes shows extended microphase coexistence; i.e., a collapsed microphase coexists with a swollen outer layer in a wide range of temperatures. We present an analytical theory that serves as a basis for understanding this phenomenon. The theory clarifies the nature of the first-order transition initiating the ordering and predicts the effect of the chain length, N, on the transition point and on the size of the critical nucleus of the ordered phase. For bidisperse brushes composed of chains of two fractions differing in contour length, the theory explains two mechanisms of ordering observed by self-consistent-field computer modeling and gives predictions for certain important features of the ordering (character of transitions, shifts of the transition points, and parameter domains of ordering mechanisms). Predictions of the analytical theory are in good agreement with the results of SCF computer modeling.