Partial mashing, or mini mashing is the bridge between extract brewing and all grain brewing. Partial mash beer brewing is the next step beyond steeping grains. What makes partial mashing one step past steeping and just short of full mashing is two things.
It is beyond steeping because you are adding mashable grains to the steep—you do not want to steep mashing grains (base malts). It is not quite all grain, because you are still using malt extract.
When you introduce base malts to steeping, you are then mashing—however—you must make sure that you adjust the beer brewing process properly to ensure you are mashing. If not, the starches will not be converted to sugars, and this will affect your beer’s clarity. You will also in effect have wasted the base malts fermentable sugars.
Assuming that you adjust the process accordingly, and mash the grains properly, you will have added fermentable sugars to your brew in addition to the added flavor and colors of the base malt(s). This also gives you a taste of all grain beer brewing, as the only difference is that with all grain, all of the fermentable sugars are obtained from the grains. With partial mash, you get your fermentable sugars partially from the grains and the rest is obtained using extracts.
Partial mashes are preferable to steeping simply because they open up more options as far as malt selection. You can use a wider variety of malts (and adjuncts), which gives the brewer more flexibility with recipes, and more control over the flavor profile of the beer.
Partial mash can also be more cost effective by replacing some of the malt extract required. One other thing that makes partial mashing attractive to extract brewers is that it doesn’t require any additional equipment to buy.
It offers more choices as far as ingredients that can be used, which opens up more possibilities for beer recipes and beer varieties. Because you are reducing the amount of extract you use, you can cut the costs of brewing. While you still have to buy grains, you can buy them in bulk to save money. Partial mash also allows you to further explore the world of beer brewing, and can be an introduction to the world of all grain beer brewing