"But" transitions are conjunctions that express contrast. "But" transitions can greatly help clarify differences and maintain reader interest.
"But" transitions are conjunctions because both premises linked by a "but" transition can simultaneously be true. However, "but" transitions indicate that two premises contrast or conflict in some way. For example, we could write "Both insects and mammals have muscles, but insects have inhibitory motor neurons (Wolf, 2014)." Even without clarification, the "but" transition implies that mammals do not have inhibitory motor neurons (which we do not).
"But" transitions are stronger than "and" transitions for at least three reasons:
1) "But" transitions express more information than "and" transitions. By expressing contrast or conflict, "but" transitions not only indicate conjunction (as "and" transitions do), but also indicate the relationship of the premises to each other. The author must specify why the two premises contrast if the reason is not self-evident.
2) "But" transitions are more interesting than "and" transitions. Conflict and change is fundamental to human storytelling (Campbell, 1991). However, conflict is not only interesting in the context of fiction writing. Emphasizing conflict can also make non-fiction communication (including scientific writing) more interesting for an audience (Olson, 2015; Luna, 2013).
3) By expressing contrast, "but" transitions may facilitate understanding and retention of information. Contrast is potentially an important facilitator of constructing long-term memories (Bilodeau, 1966). Therefore, expressing contrast may help audiences retain information.
Therefore, emphasizing conflict and contrast can be very helpful for constructing interesting, compelling, and memorable arguments.
Fortunately, the word "but" is not the only way to express a "but" transition.
More "but" Transitions
On the contrary
On the other hand
At the same time
Clearly, most arguments are not constructed solely from conflicting information. In practice, some combination of "and" and "but" conjunctions is often the most compelling way to structure a series of arguments.