Political parties do better in elections if they are united. Party loyalty is accordingly valued. A disunited party emits a bad smell to electors, internal dissension leads to splits if it goes too far. All this is a lesson emphatically taught by history, and is common knowledge.
It is however odd that party loyalty, or even the mere appearance of it, should be so prized. Cast an eye back to the 18th century when members of parliament were individually more independent, though formed into Whig and Tory factions. MPs whose seat depended on supporting the administration were known as “placemen” because they were specifically there as voting fodder. The rest, county and borough members, were in many cases effectively owners of the seats they occupied, and could not be compelled to support the government; their seats did not depend on it.