Probiotics are used as food supplements and are broadly defined as: "Live microorganisms which when administered in adequate amounts confer a health benefit on the host"
There are an increasing number of reviews and research papers on the use of Aspergillus as a probiotic in farm animal feed. It is recorded as having beneficial effects on - nutrition uptake (e.g. making nitrogen more available in ruminants), blood cholesterol levels, mineral absorption and adjusting the gut microflora. The overall purpose of feeding animals a probiotic is to increase food production, the measurement of which is feasible and achievable through measurement of food output i.e. meat weight, egg production numbers and so on.
When it comes to non-food producing animals i.e. domestic pets, the use of these products is less obvious and less measurable. These products are sold to 'make our pets healthier' in some way and also to make food more palatable - but this is not a measurable commodity for the pet-owner and there is often very little, well conducted research to support the claims. In short there is a large dose of marketing involved and much less evidence to suggest it is worth paying out sums of money for these supplements.