ACCA SBL Syllabus B. Governance - Conflicts of Interest - Notes ^{4 / 12}

Directors contracting with their own company (However, the articles may allow if disclosed)
Substantial property transactions: These need approval
Loans to directors: generally prohibited

Here a director uses information (not known publicly) which if publicly available would affect the share price
Trading in own shares with this knowledge is fraud
Directors are often in possession of market-sensitive information ahead of its publication and they would therefore know if the current share price is under or over-valued given what they know about forthcoming events.
If, for example, they are made aware of a higher than expected performance, it would be classed as insider dealing to buy company shares before that information was published.

Directors must act primarily in the interests of shareholders.
If insider dealing is allowed, then it is likely that some decisions would have a short-term effect which would not be of the best long-term value for shareholders.
This can become particularly important at times of takeovers where inside information could mean big profits for the director and not necessarily in the longer term interests of the shareholder
There is also the potential damage that insider trading does to the reputation and integrity of the capital markets in general which could put off investors who would have no such access to privileged information and who would perceive that such market distortions might increase the risk and variability of returns beyond what they should be.