Robots.txt files are an important aspect of SEO. Here are a few reasons why they are necessary and can be useful to you:

The Robots.txt file helps prevent duplicate content issues by instructing search engine bots to avoid crawling and indexing duplicate or redundant pages that have been flagged. This ensures that only the most relevant and authoritative version of a page is displayed in search engine results pages (SERPs), improving the overall search ranking and user experience.

Ideally, the best practice is to use canonical tags for duplicate content rather than robots.txt. However, both result in the same outcome.

Robots.txt allows website owners to designate specific areas or directories as off-limits to search engine crawlers. This is vital for maintaining the privacy and security of sensitive information, such as login pages or administrative sections, ensuring that confidential content remains hidden from public search results.

To avoid cluttering search engine results with internal search results pages, which are often low-quality and irrelevant to users, robots.txt can be used to block the indexing of these URLs. This streamlines the indexing process, prevents search engines from wasting resources, and keeps search results cleaner and more user-friendly.

For example, if there is a search page example.com/search, and it's configured to display the query in the URL (e.g. example.com/search?query=my+search+keywords) then this can lead to messy or irrelevant pages that shouldn't be shown in SERPs.

Robots.txt files allow webmasters to specify which types of files should be excluded from search engine indexing. By blocking files like images, PDFs, or CSVs, website owners can improve crawl efficiency and focus search engine attention on the most valuable content, potentially boosting SEO performance.

There are some instances where it may be useful to allow some types of files to be indexed, for example, if the PDFs are content pieces that are supposed to be for public consumption.

In some cases, malicious bots or excessive crawling activity can overwhelm a website's servers, leading to poor site performance and downtime. Robots.txt helps prevent server abuse by restricting access to authorized search engine bots, ensuring that server resources are used efficiently, and protecting the website's overall SEO health. It also allows for managing & optimizing the crawl budget by limiting how many pages can be crawled during a certain period of time.

Robots.txt can be used to indicate the location of a website's XML sitemap(s). This assists search engines in finding and indexing all relevant pages efficiently, which is essential for ensuring that a website's content is properly represented in search results and subsequently improving its SEO visibility.