Obituaries are a great source for tying up families from 1800- and even earlier if your ancestors were stacking dimes and lived in a city where obits. were published.
The reason being is obits. often named a great number of relatives present at the funeral. As an example:
"The mourners were: the Widow (Mrs. Roberts, Bodhyfryd); Mr. and Mrs. R. Roberts, Gwersyllt (son and daughter-in-law); Mrs. J. Roberts, Llwynmawr (daughter-in-law); Mr. and Mrs. D. Price, Maesbury (son-in-law and daughter); Mr. and Mrs. E. Roberts, Tanywen, Llangedwyn (son and daughter-in-law); Mr. D. Silin Roberts (son); Mr. and Mrs. Swannick, Maesbury (son-in-law and daughter); Mr. F. Ambrose, Penybont Stores (son-in-law); Miss D. Roberts, Gwersyllt; Miss Mary and Mr. Harry Roberts, Llwynmawr; Misses Joyce and Joan Price and Fullerton Price, Maesbury; Miss Blodwen Roberts, Tanyrywen (grandchildren); Other relatives were Mr. and Mrs. Jones, Leaton; Mrs. E. Jones, Bomere Heath; Mrs. Jones, Llanforda Mill; Mr. E. Jones, Penypark, Mrs. Davies, Fronucha, Rhiwlas; Mr. E. Jones, America Cottage."
Particularly useful were the list of other relatives, which included a 2nd cousin. By collecting obituaries for all these people and combining that with parish registers and the census, I was able to build a large and accurate tree for this family. By tracing collateral lines of your ancestors, you can really cement your own ancestry. If you only have a marriage cert. and the census to seal your connections, as JustLooking points out, you can always make an error. Whereas if you have collected wills, obituaries, civil reg. certs., church records, court records, family artefacts, criminal, tax, land, directories, immigration records, population lists, charters, deeds, military, education, employment, governmental and so on, you can prove your relations beyond a shadow of a doubt, because you have so much overlapping information.
To give a case in point. One of my ancestors was Mary Ann Ward, baptised in 1814, in Kinnerley, Salop. Her parents were Samuel Ward and Charlotte [Payne]. I found a burial for Samuel Ward in the 1820s at the appropriate abode. This matched him up to a baptism in the same parish at the same abode in 1792; and whom I thought was this Samuel Ward's grandfather named his grandson in his will at the appropriate abode. I considered it sealed. However, I decided to research all the members of this family and made notes of all the people of interest in the parish up to 1880 from parish registers. It was at this point, I noticed that there were two Samuel Wards, both living in the same place and almost the same age. On top of that, there were three other Samuel Ward's: one being the father of Samuel who was bpt. 1792 and the other two both being born in the same year to the two younger Samuels. Many wills, and parish register entries later, I discovered that I had got the wrong Samuel Ward. There were two Samuel Wards, whom were 1st cousins and by selecting the wrong one as an ancestor I could have spent a good few notes on tracing the ancestry of the other Samuel's wife, which due to he place of birth would have been much more difficult and costly.
With many families, going back into the 1700s and beyond it is quite often necessary to engage in these one name studies to sort all the people of a particular surname out. As well as helping you to not make mistakes, it can help you make breakthroughs. You may have a brick wall with your ancestor, but if you can find an uncle, a cousin, a sibling etc., you may be able to prove their ancestry.
As another case in point, I had an Edward Jones, born about 1794 in an unknown location. Since he was not alive for the 1851 census, I had no idea where he might have been born. So I began researching his children (which may people don't do) and found one living with their grandfather in the census. I now have one strand for Edward Jones going back to about 1610, because I made the decision to try and flesh out the family a bit more. Looking up the siblings of your ancestor is often a good idea, as many went to live with relatives.
Of course, this all requires 100s & 1000s of hours and often many trips to archives.
If you are new to research, I would recommend the book Ancestral Trails by Mark Herber, 2nd ed. It is a bit out of date with some records, which are now on the internet, but it has a lot of useful info on researching most important records.