These investors are cash rich companies (i.e. businesses themselves) with a long history in a sector. They are not any kind of financial company or institution. They know the sector the product, market and the business in and out. Acquisition of a small private limited company provides a lot of benefits for these companies in terms of speed of expansion and other synergies.
When to look for strategic investors? Strategic investors typically would provide more valuation. They will be also willing to acquire assets in financial distress as they understand the target business perfectly and know how to extract value from the asset. Hence, it makes a sense to approach the strategic investors in difficult financial times.