You can use a A record whenever you have to assign a specific IP to a domain name or to a subdomain.
Suppose you have the somedomain.tld domain and want to assign 10.10.0.1 IP address to your web server, then you should create an A record with "www.somedomain.tld" as Fully Qualified Domain Name and "10.10.0.1" in the value field.
From now on, all the requests for www.somedomain.tld will be sent to a server with that IP.
Basically is useful to use an A record when you have subdomains residing on various systems. If not a CNAME is better. (cf. What's a CNAME record?)
Tip: you might use a "*.somedomain.tld" A record to allow WHATEVER.somedomain.tld to be resolved to your IP, though a wildcard CNAME record is often better than a wildcard A record.
