Spreadsheet apps are great for keeping information in tables, which can become very complex (and hard to troubleshoot). The power of spreadsheets lies in the formulas you can build in cells that reference other cells, to summarize data.
Advice for spreadsheet beginners: create a separate Excel file for each task. Then within a file create a separate worksheet for subtasks so that you work only in the top left corner of every worksheet.
Commonest home use probably is as an aid to manage finances. I use it as a supplement to a bookkeeping app like iBank. I use the spreadsheet app to summarize financial data.
Spreadsheets are great for making graphs of tabled data.
Spreadsheet apps can be a useful way to make lists of stuff you own, but as someone above suggested, database programs are better for that job. Apps like TapForms are goodfor personal databases like vehicle logging, household inventories, health records, family gift preferences, books to read, movies to watch. Spreadsheets are not well suited to large databases, although experts can really make them sing.