We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
If it's a private group (and not a channel) you can download them in bulk using the official telegram app.
Go to Settings -> advanced -> export telegram data (at the end). A pop-up window will appear that asks what data you want to export. Select only private groups and UNCHECK "only my messages" in order to download all chat history. Then you can select what data to download (photos, videos, voice messages, files, gifs...). Select the path where download the data and click export.