barley malt

Yeast use it as food, so it could help with a rise. Also many of the sugars it contains are in forms used by lactic acid-producing bacteria, so it might create a better flavor in sourdoughs.

That's just from what I've read. Maybe it has more uses.

From the KA store description for their barley malt powder (basically powdered version of same stuff, AFAIK):

Diastatic malt powder is the all-natural "secret ingredient" savvy bread bakers use to promote a strong rise, great texture, lovely brown crust, and extended shelf life.

Vitamins and active enzymes in diastatic malt help yeast grow fully and efficiently throughout the fermentation period, yielding a good, strong rise and great oven-spring.

Malt also converts starch to sugar, enhancing bread's browning.

From other readings, I take it that barley malt syrup is a good addition for doughs that have to rise for a long time, to ensure that the yeast has additional "food" to support the rise.

Barley malt has a molasses-like earthy flavor too, which gives breads a different dimension, if you use enough of it.  

I often use DMP in artisan breads that require a pre-ferment, however, I use it only if the flour is not already malted. Also, the DMP goes in only after the pre-ferment is complete, in order to give the dough a second boost, as it will convert the remaining starch more efficiently.

If your flour already has malt in it, as most U.S. AP and bread flours do, there is usually no need to add additional malt. Most European flours do not contain malt or any additives. Also, Hodgens Mill AP flour does not contain malt (nor any additives). This is my U.S. flour of preference.

The malt syrup will give your dough an additional boost also, simply by adding more "food" for the yeast. The yeast really likes the high percentage of maltose in the syrup and converts it very efficiently. It's action is not diastatic, however, as it does not convert any of the starch in the flour.

You can substitute any sugar for the malt syrup; the yeast will just take longer to convert it. If you like the flavor of malt syrup, a much cheaper (and easier to find) flavor substitute is a mixture of 25% molasses, 25% honey and 50% dark corn syrup.

Cheers,

Roy