The satellite orbit information retrieved from an almanac is much less accurate than the information retrieved form the ephemeris. If during a startup period, only almanac information is available, (e.g. while the ephemeris still is being downloaded) the receiver still is able to navigate based on almanac orbits.
With almanac navigation enabled, when a new satellite reised and its reception ujst has started, the receiver might use an almanac to use this satellite in the navigation solution until the ephemeris is fully retrieved.
With an almanac only solution, the position will only have an accuracy of a few kilometers. Almanac Navigation allows much faster start up, as there is no need to wait for the completion of the ephemeris download(>18s).