The Annual Savings insights calculates the amount of additional savings needed, above any savings and investments currently planned, to prevent any projected shortfalls on or after a specified target event, which in most cases will be retirement.
A better name for this simulation would be the "Investment Need" insight, since savings are modelled as deposited into an investment (capital gains) account. Think of this model account as an investment portfolio without a dividend yield.
When run, the savings need analysis creates a special reserve account called "Needs Analysis Account". Funds are deposited into this investment account and are held until the specified target event, after which they are released for payment of expenses.
The net assumed growth rate of this taxable investment account can be viewed after running the simulation by double clicking a bar of the chart above prior to the savings target event selected for the simulation - e.g. Retirement. The chart details screen will display. Select the Investments tab. Find the "Needs Analysis Account". A growth rate will be shown for this reserve account, net of any account fees.
This growth rate is set from Dashboard View > Plan Settings > Inflation / Growth > Investment Growth Rate. The default fees, deducted from this default growth rate, are also set in Plan Settings on the Fees & Taxation panel.