I agree with the other brewers. Why use it?
A little science type stuff about maltodextrin, saccharification, conversion and dextrinization which will help when you decide to use all malt.
Because malt is tested, maltodextrin exists. Malt is tested at 145, 153 and 155F. The single infusion method is used for the tests. The temperatures are not high enough during the tests to cause starch called amylo-pectin to "melt" and the starch is left in the spent mash. Amylo-pectin begins to "melt" slowly at 169F.
In making maltodextrin protein sludge is removed from the spent mash and pH is adjusted to make Alpha happy. Some water is added to thin the mash down and then it is boiled. Boiling causes the starch to rapidly, "melt." The liquid is drained from the mash and Alpha is added when the liquid is cooled to anywhere from 149F up to the temperature where Alpha denatures. After that an acid is added and a few steps later mud forms. The mud is dried and powdered and maltodextrin is produced. Brewers Malt Extract and Bakers Malt Extract are bi-products of three tests and maltodextrin is mixed in. Nothing is thrown away.
Malt is tested because it is very inconsistent. A spec sheet comes with each sack of grain and the data listed on the sheet comes from the tests. The sheet is used for determining the quality of malt. It's not a bad idea to learn about the chemicals, acronyms and numbers that are listed on the sheet. It is some interesting stuff.
Like RM mentioned. When malt is boiled heat resistant starch "melts." Amylo-pectin is located at the tips of the kernel and it rapidly enters into solution when mash is boiled. The starch is the richest starch in the kernel because it contains limit dextrin. A and B limit dextrin are tasteless, non-fermenting types of sugar responsible for body and mouthfeel. During the decoction method mash is boiled and amylo-pectin rapidly enters into solution. The boiling mash is added back into the main mash and dextrinization occurs when Alpha liquefies the starch chain at 1-6 links. When the starch liquefies limit dextrin is released. At the same time saccharification is going on when simple starch, amylose is liquefied by Alpha at 1-4 links.
Alpha is responsible for liquefaction, saccharification and dextrinization.
Beta is responsible for conversion at a rest somewhere around 140F. During conversion, Beta converts glucose into complex types of sugar, maltose and maltotriose. Yeast loves glucose a lot, it doesn't love complex sugar, too much. So, when a Beta rest is used second fermentation is required because during second fermentation a reverse conversion occurs. Since, yeast rips through all of the simple stuff during primary fermentation, the only stuff left is complex sugar which yeast doesn't love, too much. So, due to the Great Magnet knowing that sooner or later apes would figure out how to produce ale and lager, He/She/Alien equipped yeast with an enzyme that deals with complex sugar just so we could make ale. Friggen way cool! Now, here's what goes on. Yeast absorbs maltose through the cell wall during second fermentation and the enzyme converts maltose back into glucose. The glucose is expelled back through the cell wall and yeast uses the sugar for fuel. During second fermentation gravity reduces closer to expected FG. During the aging phase the same thing happens to maltotriose and natural carbonation occurs and FG is hit.
I don't believe Beta plays a significant role in producing Carapils or Crystal. The malt did not pass brewers or distillers specs and that is why it became a specialty malt. When malt contains Beta, the malt would have been sold for making Ale and Lager. Cara and Crystal share a bench with rancid malt. Sometimes, the malt will go 180 degrees opposite of what the literature mentions about it.