Advantages of PDF/A over TIFF
The TIFF image format was the first widely used format for digital archiving. TIFF is a raster image format that promises the same guaranteed visual appearance of the document as the original document. However because it is an image format it is missing the ability to include vector content such as shapes, gradients and vector fonts that are available in PDF. Not only does vector content more accurately describe the original document, it is often a smaller document size than raster content, which can be considerable when archiving a large number of documents.
Another advantage that PDF/A has over TIFF is that unicode can be included to ensure document text is searchable and extractable. With TIFF documents, you will need to use a robust OCR engine in order to accurately search and extract text, which is not always 100% accurate.
Lastly, the PDF/A format can accommodate embedded digital signatures and certificates, enabling the ability to verify that the PDF has not been altered from the instance it was certified, which can be important for the legal admissibility of a document.