I will try to be as objective as possible in the answers:
What is a TXT input for?
A TXT record (text record) is a type of resource record in DNS, which contains text information from sources outside your domain, used to associate arbitrary text to a host, such as readable information about a server, network, etc. You can use TXT records to prevent phishing and Spamming.
How to read the TXT configuration of a given domain?
As José Maia said, to read the configuration, it is done in the same way as any other DNS lookup, i.e. Nslookup.exe -q=TXT example.com or Dig -t txt example.com.
Who cares that I have a TXT configured in my domain?
The domain host maintains the DNS records, which direct Internet traffic to your domain name. When you add a TXT verification record DNS records from your domain host. if there is any verification (for example by Google to use services such as G Suite), your domain ownership is confirmed.
What are the advantages of knowing this in day-to-day programming?
The main focus is to have an overview that you have a TXT record confirms that you are the owner of your domain, for example when Google or Office365, etc, see the record, the ownership of the domain is confirmed.
How many TXT records can I have per domain?
A domain can have multiple TXT records associated with it, provided that the DNS server implementation supports it. But you cannot have more than one TXT record for the SPF (Sender Policy Framework) of a domain. If the domain has more than one SPF record, you will receive email errors, as well as spam sorting and delivery issues.
Note: The verification log does not affect your website or email.
TXT is free entry, serves anything you want to implement. A common usage (which gave rise to the question, in network chat) is SPF, but this is only one of many uses.
– Bacco