Don't worry, it's not a stupid question ?
You can definitely build a custom search feature and get great results with that, but things tend to get a little more complicated if your site contains a larger amount of fields, and particularly if you're using Repeater, RepeaterMatrix, PageTable, or other types of fields with repeatable content — or perhaps page reference fields gluing content from different pages together. In other words content that isn't technically (directly) part of the page, but needs to be tied with the page in the context of site search.
Although in many cases you could no doubt include all those fields in your search queries, this can result in pretty complex queries, and such queries also tend to become inefficient. Generally speaking the more fields you join in the query, the more complex the resulting SQL query will get, and the more time and resources it'll take to process. Not a great thing for scalability.
First version of SearchEngine was really just an easy way to reuse bits and pieces of code that were developed over time to mash field values together so that they could be searched more efficiently. Soon after along came the markup generating parts (which now make up a notable portion of the module), a set of features for automatically filtering and sanitizing queries and processing the index, JSON output option (mostly for AJAX requests), indexing support for field types requiring specific handling (core ones as well as third party), etc.
From the initial post in this thread:
These days in my projects I install SearchEngine, set it up, and trigger the render function. The module takes care of everything else and "just works". In a nutshell SE bundles most of the stuff a typical site search will need into one package, and tries to do it efficiently and following best practices ?
Hope this answered your question!