Liandry caters to a very specific subset of champions. These champions excel at whittling down their opponents from afar before the teams engage for a fight. They are characterized by low cooldown long range "poke" spells, or with trap abilities left waiting in common paths.
Teemo, for example, loves Liandry. Toxic Shot is already a DoT effect. When combined with his mushrooms alone he gets a huge damage increase in the fiend. When he gets a Rylai for some survival, he gets an even bigger damage increase, while also gaining increased damage off of Blinding Dart.
Depending on your team comp, Nid is a good candidate for Liandry as well, for very different reasons, as a situational item choice. Javelin Toss has a really low cooldown and really long range. Bushwack traps are single target, and have a fairly debilitating % Reduction debuff, while revealing you for more Javelins. Literally the only thing missing from this combination is Rylai to make both of these whittling abilities slow for double the Liandry damage.
Originally Posted by rainzer
some people really insist on forcing the Rylai's/Liandry's (super expensive man) combo build.
Riot thought about this specifically while designing Liandry. It made even more sense when they built Liandry on Sejuani while testing it.