When I do a dig on a hostname, in our network, it doesn't not give me the IP address, but when I add the domain name it does work or when I do a dig +search
Why is the domain name a prerequisite for DIG to resolve the hostname to IP?
Because dig is working as designed.
snip from man page:
+[no]search
Use or do not use the search list defined by the search list or domain directive in the /etc/resolv.conf file (if any). The search list is not used by default.
In short, do not confuse dig with nslookup