Sri Lanka has undergone a huge boom in tourism over several years. The cost of living there is very affordable, its legendary, 2,000- year-old culture is welcoming and there are many UNESCO World Heritage sites on the island. A guide will enable you to explore the country's heritage under the best possible conditions.
Travel in Sri Lanka gives you a chance to explore places and landscapes which are strongly associated with Sri Lankan Buddhist culture. Although the views are very beautiful and the treks and safaris very enjoyable, the experience is magnified by being able to understand the spiritual and historical context of the places you visit, as well as their importance to the locals. The island's role in trade and as a hub in the Indian Ocean are also deeply ingrained in the architecture, cuisine and culture of the country. There are so many features that only a local could explain it all to you.
Lastly, it is most unlikely that you speak Sinhalese or Tamil, so a guide will be a great help as a translator in any dealings and discussions with locals, whose English can sometimes be a bit basic.
Sri Lanka has been a very safe and peaceful destination since the end of the civil war. However, it is still a country where scams are developing at the same rate as tourism. A government-approved local guide will mean you don't have to search for a new person for every tour and is a guarantee of professionalism (unlike the locals who declare themselves to be tourist guides without the appropriate certification).
He will also be able to help you haggle for your souvenirs, avoid over-charging in restaurants (most likely linked to your status as a soft tourist target) and warn you about areas where there is an increased risk of pickpockets or mugging, particularly in Colombo and Kandy.
It's often great to eat in local restaurants when you're on holiday, rather than the less authentic options aimed at tourists.
It's also easier to see elephants bathing very early in the morning or in late afternoon. Hints like this, provided by your guide, will ensure you get the most out of your days in terms of exploration and experiences and will help you organise your trip in the best possible way.
Unless you're planning to spend the whole of your stay in Sri Lanka lazing around on the beach, I really do recommend that you hire an approved guide to help you discover this unique culture.