Can I compare this year's sales with last year's so I know whether my agency is actually growing?

Yes, you can compare one period against another, as long as the system is holding your bookings for both of those periods. Most reporting screens let you pick a date range and then put it side by side with the same range from the year before, so you are looking at a like for like picture rather than guessing from memory.

Once you have that, you can usually break the comparison down in the ways that matter to you. You can look at how many bookings came in, what you sold in total and what you earned after supplier cost, and then split that by flights, hotels, packages and other services. You can also see it by staff member, by branch, by sub agent or by destination, which is often where the real story sits. Two years can look similar overall while one destination has quietly collapsed and another has grown to cover for it.

The honest limit is that a comparison is only as good as the history behind it. If you started using the system part way through a year, there is nothing to compare the earlier months against until a full year has passed. Older bookings sitting in a previous system or in spreadsheets will not appear unless they are brought across at setup, and when they are, some of those records can be incomplete, so your first year of comparison may look uneven. Anything handled outside the system, such as a cancellation agreed directly with a supplier, will also be missing from the figures. How deep the reporting goes can vary with your plan.

Before you rely on it for decisions, check which comparison reports your plan includes and whether your past bookings can be loaded in at the start so you have a proper baseline to measure against.