Question

Why can aggregation of all a taxpayer's activities be advantageous? Why would it be disadvantageous?

Answer 1

Answer-

Advantages of Aggregation for tax payers

Aggregation results in businesses that would be treated combined into one business rather than as separate for purposes of calculating the Qualified Business Deduction (QBI) deduction.
QBI is the net amount of qualified items of income, gain, deduction and loss with regards to any trade or busibness.

For taxpayers who are qualified for aggregation allows the wages and capital limitation of Sec. 199A to be calculated for the combined business rather than treating as a single entity.
The aggregation may allow a taxpayer to claim a greater QBI deduction when combined for the wages and capital limitation rather than when treated separately.

Disadvantages of Aggregation for tax payers

If a taxpayer has one business that generates negative QBI, then the QBI from each business that are positive proportionally decreaes the positive QBI and gives loss.  
Losses in one or more businesses reduce the positive QBI before the wage limitations are applicable.

The aggregation of loss will not help the tax payer to benefit from the W-2 wages and qualified property from the business that are loss making.