In the special case where transmitter output impedance, transmission line characteristic impedance, and antenna impedance are all the same, for example 50 +j0 ohms, then everywhere on the transmission line there is no standing wave and the line impedance (live impedance during operation) will equal the characteristic impedance of the line.
Assuming that transmitter and characteristic impedance of the transmission line are matched, as soon as there is a mismatch at the junction of transmission line and antenna, there will be a standing wave on the line, and this standing wave causes the line impedance at each point along the transmission line to change as a function of the distance from the antenna.
This means that at each point along the line, when there is a standing wave present, the SWR will be the same but the impedance will be different. The SWR is determined by the ratio of transmission line impedance to antenna impedance, the line impedance at each point along the transmission line is determined by the ratio of transmission line impedance to antenna impedance and the distance from the antenna.
An antenna tuner, depending on it's configuration, allows reactance of varying amplitude and phase to be added in parallel or series with the transmission line. When installed at the transmitter end, it allows reactance present at that end of the transmission line to be cancelled out by adding reactance of the same value but opposite polarity. This also allows a non-reactive impedance which isn't 50 ohms eg: 35 + J0 ohms to be transformed to exactly 50 + j0 ohms by adding the appropriate amount of positive and negative reactance in series, or in parallel if the impedance is higher than 50 ohms.
Using an antenna tuner at the transmitter end is obviously much more practical than connecting at the antenna, the disadvantage of doing this is that the tuner doesn't remove the standing wave from a transmission line, and I²R losses will be higher due to the increased current caused by the presence of the standing wave.
In fact the conditions in terms of impedance on a transmission line are determined by what is connected at both ends. This is described as boundary conditions in transmission line theory. If there is a standing wave then there can be reflection also at the transmitter end if transmitter output impedance and line characteristic impedance are different.
If an antenna tuner is adjusted so it's output is anything other than 50 ohms, this will result in current reflected back from a mismatched antenna to be re-reflected at the antenna tuner, resulting in complex multiple reflections which also affect the impedance at every point along the line.
Using an antenna tuner at the antenna end results in no standing wave on the line and is better for this reason only. Often the increased I²R losses are negligible and only need be considered when using high power.
Hope that helps !