Custom spreadsheet solutions which survive long after the developer has "left the scene", are often those which users accept because they discover it _ among other benefits _ makes their work quicker and easier to do. That is why the best person to develop such solutions tend to be one who works in that area, and is therefore familiar with the way the manual system operates. S/he would have an "insider's" perspective of the best way to introduce automation other users will readily adopt _ even as it solves the identified problem(s).
In addition, visualise it having dynamic query/report drop menu interfaces, and a variety of custom buttons: for navigation(within and between worksheets), printing, data export (as PDF documents or spreadsheet files), saving, auto_data filtering, auto_charts plotting, auto_backup of files, user login authentication (for documents with sensitive or confidential content) etc.
At times, they would prefer to click a button that says "Print ABC", or "Print XYZ" report, instead of having to crawl all over the huge spreadsheet(and get "lost" every now and then), to highlight and print different report pages. Using a custom built data entry form to make data entries into 14 different cells in different parts of a table(at the same time/with one click) would, for them, be "heaven" compared to making the entries one at a time.
You can choose to learn how to do it yourself _ or call in someone(an Excel VB Solutions Developer for instance) who knows how. Again, this would be dependent on your purpose, how proficient you are, and/or how much time you have at your disposal. Ultimately, even if someone develops a custom application for you, it should happen with YOUR guidance at every point to ensure that it does EXACTLY what you want it to do, and that you can easily _ with your developer's support/coaching _ learn how to make modifications to it in future, without needing to call back your developer.